Facies Patterns of a Tectonically-Controlled. Upper Triassic Platform-Slope Carbonate Depositional System. (Carnian Prealps, Northeastern Italy). Andrea Cozzi ...
FACIES
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Erlangen 2002
Facies Patterns of a Tectonically-Controlled Upper Triassic Platform-Slope Carbonate Depositional System (Carnian Prealps, Northeastern Italy) A n d r e a Cozzi, Z 0 r i c h
KEYWORDS:PLATFORM-TO-BASINTRANSITION - SYNSEI)IMF,NTARY I~XTKNSI()NALTECIONICS CARNIAN I-'REALPS(NORTHEASTERN ITALY)- DOLOMIAPRtNCIPALE- F(IRNI DOLOMITE UPPERTRIASSIC Contents
Summary 1 Introduction 2 Geological setting 3 Sedimentary facies across the platform-to-basin transition 3.1 Inner platform facies 3.1.1 Depositional Environment lnterprelatmn 3.2 Outer platform facies 3.2.1 Depositional Environment lnterprctauon 3.3 Margin facies 3.3.1 Depositional Environment Interpretation 3.4 Upper slope facies 3.4.1 Depositional Environment lnterprctauon 3.5 Lower slope facies 3.5.1 Depositional Environment lnterpretauon 3.6 Basinal facies 3.6. l Depositional Environment lnterpretatmn 4 The Dolomia Principale deposilional model 4.1 Middle Norian climate and fossil biota of the I)P margin facies The Forni Dolomite depositional model: reconstruction of the original slope geometry 5.1 Evolution of the geometry of the slope 5.1.1 Stage 1: Early-Middle Alaunian 5.1.2 Stage 2: Upper Alaunian-Sevatian 5.2 Discussion 6 Middle-Late Norian extensional tectonics control on lhe platform-slope depositional system Conclusions References SUMMARY
Upper Triassic (Middle-Upper Norian) shallow-wa ter carbonates of the Dolomia Principale and its deepwater counterparts (Forni Dolomite) have been studied in the Carnian Prealps (northeastern Italy). The Dolomia Principale was a storm-dominated carbonate platform; in the Mt. Pramaggiore area, along a well-preserved 3.5 km-long platform-to-basin transition, the inner platform facies of the Dolomia Principale, characterized by mscale shallowing upward cycles, give way seaward to open marine storm-dominated shallow subtidal lagoon deposits with frequent hardgrounds and evidence of microbial stabilization of the bottom sediment. The margin of the Dolomia Principale platform was colonized by meter-scale stromatolites and serpulid-microbial mounds
thai thrived due to tbe local highly stressed environmenl, characlcrized by drastic salinity fluctuations and turbid waters, that excluded the Upper Triassic coral-sponge communities. The Forni Dolomite slope-basin complex was characlerized by an upper slope facies with debris flows, megabrcccias, Iurbidilcs and serpulid-microbial mounds. The lower slope and basinal facies show thinning and fining trends. After restoring the original geometry of the slope, the depositional angles o1 the clinoforms range between 11 and 36 degrees, rellccting closely the coarse-grained character o[ the Forni Dolomite slope complex, which can be interpreted as a slope apron lhat. as a model, can be extended to steeply inclincd carbonate slopes. The onset of synscdimentary extensional tectonics at the Middle-Late Norian boundary' affected the platform-slope depositional system via: 1) localized inner platform collapses and the formation of an intraplatform anoxic depression at Mt. Valmenone. 2) a switch from platform lateral progradation during the Middle Norian to vertical aggradation in the Late Norian, reflected in an increase in plat fornl relief, steeper foreslope angles and coarser-grained slope facies, and 3) controlling the spatial orientation of the margin of the Dolomia Principale.
1 INTROI)UCTION The Upper Triassic Dolomia Principale Fm in the Southern Alps and Apennines (Italy), its equivalent in the Northern Calcarcou,,, Alps (Hauptdolomit, Zankl, 197 I: Fruth and Scherreiks. 1982, 1984) and Transdanubian Central Range (F6dolomit. Haas, 1993; Balog et al., 1997) is well known for its shal low-water carbonate deposits that were bordering the Neotethys Ocean at the end of the Triassic. Although in most cases the Dolonfia Principale appears as a monotonous succession of meter-scale peritidal shallowing Ul)ward cycles (Boscllini, 1965, 1967:. Bosellini and Hardie, 1988), the Southern Alps (Lombardy, Cirilli and Tannoia, I988; Trombetia and Claps, t 995; Berra and Jadoul, 1996) and the Apennines (Climaco el al., 1997) have good exposures of the platform margin and slope limies equivalents. The Upper Triassic terrains of the Carnian Prealps, h)caled in the eastern part of the Southern Alps, have many completely preserved plalform-to-basin Iransects, from the
Address: Dr. A. Cozzi, Institut fiir Geologie, ETH Zentrum, Sonncggstrassc 5, C1-t-8(J92 Ztirich, c-mail: andrea.cozzi(~erdw.ethz.ch
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Fig. 1. (a) Schematic location map of the Carnian Prealps of northern Italy. (b) Simplified geologic map of the Catalan Prealps. Note the E-W trending alpine overthrusts, the location of the study area and the abundance of well-preserved platform-to-basin transitions in the Norian terrains. Dolomia Principale inner platform to the adjacent relatively deep water Carnian basin (Fig. lb). This is why the Carnian Prealps provide the opportunity to investigate in detail lateral facies changes from inner platform deposits to platform margin and slope (Cozzi and Podda, 1998; Cozzi, 1999), thus allowing to reconstruct in detail the depositional environments of both platform and slope. Moreover, the Carnian Prealps during the Late Triassic recorded both the waning rifting phases connected to the westward opening of the Neotethys Ocean, and the initiation of rifting controlled by the future opening in the Middle Jurassic of the central Atlantic (Gactani et al., 1998; Cozzi, 2000). As a consequence, it is possible to investigate the effects of synsedimentary extensional tectonics on the shallow-water carbonate platform-slope development through time. In this paper the sedimentary- facies architecture of one of the most complete platform-to-basin transect in the Carnian Prealps will be described, and a depositional model for the Dolomia Principale shallow-water carbonate platform and for its adjacent carbonate slope and basin will be discussed, taking into consideration the role played by synsedimentary tectonics on the sedimentary facies organization and platform development. 2 GEOLOGICAL SETTING The Carnian Prealps (northern Italy) are part of the eastern Southern Alps (Fig. la), the southward-thrusted terrains belonging to Apulia (or African Promontory, Biju-Duval et al., 1977; Channell et al., 1979; D'Argenio et al., 1980) that together with Africa collided with Europe to generate the Alps. The Upper Triassic to Neogene
sedimentary successions that outcrop in the Carnian Prealps have experienced intense shortening due to major tectonic south-verging overthrusts that strike roughly east-west through the region (Fig. lb). The study area is centered on the Mt. Pramaggiore massif, located in the northwestern part of the Carnian Prealps (Fig. lb). There the only important tectonic line is the Mt. Dof-Mt. Auda overthrust, located to the south of Mr. Pramaggiore, with no effects on the succession studied. A minor south-verging overthrust cuts through the Mt. Pramaggiore area, but its effects are negligible. In the Carnian Prealps, during the Late Carnian-Norian, deposition occurred on a shallow-water mixed carbonate-siliciclastic ramp (Monticello Fm, Carulli et al., 1997), overlaying the evaporitic units of the Rain Fm (Fig. 2). During the Early Norian, possibly due to a first extensional tectonic phase (Podda and Ponton, t997), in the slowly subsiding areas sedimentation continued in a shallow-water carbonate platform environment resulting in the deposition of the Dolomia Principale Fm (DP, Boselllini, 1967; Bosellini and Hardie, 1988). In the rapidly subsiding areas deeper water conditions prevailed and the Forni Dolomite Fin was deposited (DF, Dalla Vecchia, 1990; Carulli et al., 1997). Biostratigraphic data tYom both platform (benthic forams) and slope (conodonts) deposits (Cozzi and Podda, 1998; Cozzi, 1999) assign the part of the DP and DF outcropping in the Mt. Pramaggiore area to the Alaunian-Early Sevatian (Middle-Late Norian), in good agreement with conodont assemblages recovered within the DF in sections only 10's ofkm to the east of the study area (Roghi et at., 1995). At the end of the Middle Norian a second and more intense extensional tectonic phase segmented the carbonate
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Facies I, 2 and 3 belong to the shallow-water carbonate platform of the DP, while facies 4, 5 and 6 arc part of the deep-water deposits of the DF (Fig. 4b). This distinction is based on the fact thai predominant in .situ production of sediment occurred on lhe platform flat top and in line tnargin aret,, as it is in most modern and ancient shallow-water carbonate systems. ThereI'ore, the first three facies of the DP correspond ahnost completely 1o the 'carbonate faclory" w'hile the second three pertain tc~ the DF, where gravity-driven redcposition processes were dominant. In the field, the Norian outcrops ollhe DPand l)F make the identification of the plalform break Iidrly simple, while it is difficult to identify the precise transition between the slope complex and the basmal scdilncnts. ]his is also the reason why upper slope, lower slope and basinal Iacies have been lmnpcd together in the DF.
3.1
Fig. 2. Upper Triassic-Early Jurassic stratigraphy of the Carnian Prealps. Shallow-water carbonates to the left and deep-water basins to the right. Time scale after Marcoux ct al. (1993) and Gradstein et al. (1995).
platforms into fault blocks, causing extensive drowning of the DP platform (Carulli et al., 1998). On the horsts shallowwater environments persisted with the calcareous deposits of the Dachstein Limestone, while in the grabens the basinal Chiampomano Limestone accumulated (Ponton and Podda, 1995). During the entire Rhaetian normal faulting conti nued until the northernmost part of the Carnian Prcalps became gradually the site ofhemipelagic deposition (Soverzcne Fro) in the Late Rhaetian-Early Liassic (Fig. 2). This continuous late Middle Norian-Early Liassic extensional tectonic activity in the Carnian Prealps reflects the plate scale rifting connected to the westward opening of the Neotethys, aborted in the Late Triassic, and the one that initiated in the Rhaetian and led to the opening of the Ligurian-Piedmont ocean, to the west of the Carnian Prealps, and the Central Atlantic in the Middle Jurassic (Gaetani ct al., 1998; Cozzi, 2000).
3 SEDIMENTARY FACIES ACROSS TIlE PLATFORM-TO-BASIN TRANSITION
Continuous and nearly undeformed outcrops from the DP platform to the DF basin are found in the Mr. Pramaggiorc area (Fig. 3) (Cozzi and Podda, 1998; Cozzi~ 1999). Along a 3.5 km-long natural section (Fig. 4a) six sedimentary facies are observed: 1) inner platform; 2) outer pIat|orm; 3) platform margin; 4) upper slope; 5) lower slope; 6) basin.
Inner p l a t f o r m facies
The inner platform facies of the DP was compt'ehensivcly studied by Bosellini (1965, 1967), who interpreted this facies as being of peritidal origin. Bosetlini and Hardie (1988)revised the general scdi~200 m mentology of the DP deposils, distin,guishing between two types of cyclicity in the Dolomites and Vencto region: ,,,hallowing upward peritidal cycles and "diagenetic" cycles, the lalter being capped by meter scale tepee antiforms. Shallowing upward peritidal cycles of the type described by Bosellini and Hardie (1988) arc a predominant stratigraphic feature of the western parl of the present study area. This depositional style is gradually lost loward the cast (Mr. Pramaggiore) where the lateral transition lo Ihc outer platform facies occurs. The average thickness olthc peritidal cycles falls in the range of I to 1.5 m, with a sharp and erosive upper and lower boundary to each cycle. Wilhin each cycle, the transition between the different lithofacies is commonly gradational, following Walthcr's law lk)r the succession of sedimentary facies (Middleton, 1973). From the bottom to the top of a cycle, superimposed one o v e r the other, three lilhofacies are distinguished: I ) fiat pebble conglomerale, 2) bioturbated grainstone-mudstone and 3) planar and wavy laminites. Sharp contacts between lithotacies 2 and 3 do occur in some cycles, and lithofacics I is not always present at the base of the cycles. Lidmfacies I is characterized by flat platy clasts, 1-5 ctn in length, that have both rounded and sharp edges (P1. 26/1). The pebbles arc made of Iknestral laminites eroded from the top of the underlying cycle. The bulk of a single sedimentary cycle (lithofacies 2), with an average thickness of 50 to 100 cm. consists of an alternation of: 1) coarse grained gl'ainstone-p-~ckstonewackcstonc with minor breccia (era-size clasts) overlying erosive surfaces, and 2) bioturbated peloidal-intraclastic wackcstonc, lnlcnse bioturbation is characleristic of this lilhofacJes (PI. 26/2); the high organic mailer c,,mtent generates a dark-grey color and a hydrocarbon smell at breakage.
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Fig. 3. Detailed geologic map of the Mt. Pramaggiore area showing the distribution of the sedimentary facies mapped.
The upper part of the sedimentary cycle (lithofacies 3) consists of alternating mudstone and grainstone-packstone layers (P1.26/4) with ram- to cm-thick horizontal Io wavy and crinkled laminae. The average thickness of the laminae is typically between 0.5 and 2 ram, but locally reaches 1-4 cm. Within the coarser beds a crude vertical normal grading is observed with a transition from grainstone into mudstone from base to top. Mud-cracks and polygonal-crack patterns are rare, but sheet cracks and laminoid-irregular fenestrae are commonly observed. They can be laterally continuous over a decimeter distance, lined by an isopachous rim followed by coarse sparry cement and commonly infilled by a fine silt-sized crystalline mosaic (cf. the 'vadose silt' of Dunham, 1969), generating geopetal fabrics. Bioturbation can also be present in the lower part of the laminites where the layers are heavily disrupted. One of the most distinctive and interesting features of the laminae, observed under the petrographic microscope, is the occurrence of tubular molds (Microtubus communis, Fliigel, t981), 10-30 gm in diameter and up to 100 I.tm long, now entirely filled by marine cement. They can be present either as isolated filaments vertically cutting through the laminae orin bundles (P1.26/3). In the latter the vertical free-standing bundles (0.5 cm in length on average) are made of tubes (2030 ~m across, several hundred gm long and with walls 5-10 gm thick) twisted along their longitudinal axes, while in the
intercolumnar spaces wackestone-packstone layers with irregular fenestrae occur stacked one upon the other (PI. 26/3).
3. l.l
Depositional Environment Interpretation
The regular vertical repetition of flat pebble conglomerates, bioturbated wackestones-grainstones and planar to wavy laminites is best explained by a classic tidal flatlagoon depositional environment. The classic modal cycle of the DP inner platform facies is remarkably similar to the Holocene succession in Florida and Bahamas tidal flats (Ginsburg's 'lag-trap-cap' cycle, Hardie, 1986). The flat pebble conglomerate, at the base of the DP cycles, represents the erosive 'lag' due to the first marine transgression over the laminites lithofacies. In the bioturbated grainstone-mudstone lithofacies of the DP, coarse sediment is abundant and the character of the undulatory layering and graded beds could represent the sedimentary record of storms. The DP cycle laminite cap closely resembles the upper part of modern carbonate tidal flat depositional systems (e.g. Andros Island and Sugarloaf Key) (Shinn et al., 1969; Hardie and Ginsburg, 1977). In the DP sediments the dctrital origin of the laminae, their erosive character, crude normal grading and lateral pinching out, together with the occurrence of current lineations
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Fig. 4. (a) Photomosaic and sketch of the natural cross-section at Mr. PTamaggiorc. Note the contint, ous exposures of Ihc platformto-basin transition from the Dolomia Principale Fm (NW) to Fomi Dolomile Fm (SE). Fo~ thc exact localion of the field or view see the map of Fig. 3. (b) Schematic representation of the sedimentary facies that characterize the platform-to-basin transition in the Mr. Pramaggiore area. For exact lateral facies widths refer to Fig. 9.
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features, all point to a storm layer origin similar to that of the supratidal laminites of Andros Island (Hardie and Ginsburg, 1977). The vertical transition from planar and crinkled laminae to planar mud-sand layers alternations (Pl. 26/4) could represent a transition from the high algal marsh subenvironment to the levee subenvironment by comparison with the modern analogs (Hardie and Ginsburg, 1977). The paucity of mud-cracks in the laminites lithofacies could be the result of either lack of prolonged exposure or of the binding action of microbial mats like Schizothrix today in the modern environments. Close similarity exists between the 'palisade' structure of Hardie and Ginsburg (1977), found today in inland fieshwater marshes on Andros Island, and the radiating calcified bundles oftubes found ill the DP laminites (see PI. 26/3). By analogy, this feature of the Upper Triassic laminites of the DP could be used as a very strong indication of freshwater input on the top of the tidal flats, indirectly suggesting a tropical humid paleoclimate at the time the DP was depositing. The cements, observed especially within the bioturbated packstone-mudstone and planar to wavy laminites lithofacies, reflect early marine cementation by fibrous and microcrystalline marine precipitates followed by a later stage burial cementation occluding the remaining pore space. Although the 'laminite caps' of the DP cycles were deposited in an upper intertidal-supratidal setting, no vadose fabrics have been observed, comparable to their paucity in today modern peritidal environments (Hardie and Ginsburg, 1977). The fossil fauna of the DP inner platform facies can be described as being restricted, based on the low species diversity (Fig. 6). The previous studies on the Dolomia Principale inner platform facies confirm their restricted character (Bosellini, 1965, 1967; Bosellini and Hardie, 1988), which conform well with modern analogs (e.g. Garrett, 1977).
Plate Fig. 1. Fig. 2. Fig. 3.
Fig. 4. Fig. 5. Fig. 6.
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3.2 Outer platform facies This facies represents the transition between the inner platform facies and the margin facies of the DP and it shows characters common to both of them. The most notable feature of this facies belt is the progressive loss of the well defined cyclicity typical of the inner platform facies as the margin is approached. The transition zone between inner and outer platform facies is characterized by interfingering of their lithofacies over a lateral distance of 10s of meters. Two different lithofacies are representative of the outer platform facies: dasycladacean packstonegrainstone (1) and lithoclastic peloidal wackestone-packstone (2). Lithofacies 1 is rich in the dasycladacean alga Griphoporella curvata (GiJmbel 1872, Barattolo et al., 1993). The deposits are organized into beds 5-20 cm thick that usually overlie and scour into the underlying clotted peloidal wackestone layers (lithofacies 2) (Pl. 26/5). The algal stems are intensely cemented by fringes of early isopachous marine cements and can be almost completely encrusted by microproblematica micritic envelopes (PI. 26/6). Lithoclastic-peloidal wackestone-packstone (lithofacies 2) occur throughout the outer platform facies, but increase in abundance towards the inner platform facies. Closer to the margin oncoids, serpulid tubes and small digitate stromatolites are found. An important characteristic of this lithofacies is the ahnost ubiquitous presence of small (200-500 gm) and large (>500 ]am) fenestrae. The small cavities are associated with poorly sorted packstones in which the grains are lithoclasts and peloids (P1.27/1). The poor sorting, packing and irregular shape of the grains determines the outline of the interparticle pores. Some ram-long cavities with a preferential planar orientation are found beneath large lithoclasts and mollusk shells, a relationship that indicates that they are shelter pores (PI. 27/1). The large lithoclasts are broken up pieces of mm-thick cemented layers which occur at irregular
Mt. Pramaggiore (Carnian Prealps, northeastern Italy), inner and outer platform facies of the Dolomia Principale, Middle-Upper Norian. Disconformity at the top of a shallowing-upward peritidal cycle; note the presence of flat pebble breccias with clasts eroded from the underlying planar and wavy laminites; marker pen is 15 cm long. Bedding plane view of intense bioturbation pattern in the subtidal grainstones-mudstones lithofacies; lens cap is 5 mm across. Well preserved bundles of vertically-branching tube molds within the laminite lithofacies of the DP. Note the large cavities on each side of the filaments partially filled with peloidal siltstones. Pillars made of filament bundles parallel one another giving a 'palisade' aspect to the rock; scale bar is 500 ram. Polished slab of top of DP peritidal cycle; note the vertical transition from wavy and crinkled laminae at the base into planar discontinuous laminae at the top; scale bar is 1 cm. Erosional surface between dasycladacean packstone-grainstone and clotted peloidal wackestone lithofacies; scale bar is 1 cm. Thin section photomicrograph of dasycladacean grainstone encrusted by microbialites; note the micritic envelopes surrounding the large bioclasts (small arrows); scale bar is 500 ~tm.
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intervals throughout the rock, in some cases 3-5 cm apart (PI. 27/1). The main characteristic of these cemented layers is the presence of a micritic film (10-20 lain thick) that coats framework and bioturbation cavities and grains. Cementation increases towards the top of the surface which always has a sharp termination: no borings have been recognized. The larger cavities (0.6-1 cm) with circular cross-sections are interpreted as burrows. Microproblematica are often found associated with both types of fenestrae. Rich horizons of Thaumatoporella Parvovesiculiflera (Raineri) are present, some large cavities containing thaumatoporellids in their center. Other problematica occur as bundles of tubes that envelope mud and silt particles, creating shelter pores, giving the rock the peculiar aspect of an assemblage of irregularly distributed clotted areas and others where bioturbation was intense (PI. 27/3), concentrated preferentially around and within the filament molds bundles. Problematic disruption features resemble sheet-cracks or 'zebra rocks' (Fischer, 1966). Mudstone layers 0.5 mm thick are fragmented along irregularly shaped mostly horizontal cracks filled with cement (PI. 27/2). The layering in the mudstones is defined by alternating laminae of micritic mudstone (with filament molds) and peloidal mudstone. Between the peloidal mudstone and the overlying micritic mudstone laminae is a coarse fbrous marine cement (PI. 27/ 2). These cement-filled cracks connect with each other through irregular vertical apertures. On a macroscopic scale the pattern of sediment and cement alternations is continuous for a couple of meters, having a convex upward relief. Most commonly fibrous isopachous marine precipitates (now calcite or dolomite) and a microcrystalline (micrite) cement are present in this lithofacies, preserved due to fabric-retentive dolomitization. 3.2.1
Depositional Environment Interpretation
The outer platform facies is interpreted as deposited in an open marine storm-dominated back-margin lagoon. The cemented layers within the lithoclastic peloidal
Plate Fig. 1. Fig. 2.
Fig. 3.
Fig. 4.
Fig. 5.
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wackestone-packstone lithofacies are interpreted as submarine hardgrounds, similar to modern occurrences in the Great Bahama Bank (Taft et al., 1968; Shinn, 1983; Whittle et al., 1993) and Persian Gulf (Shinn, 1969; Taylor and Illing, 1969; De Groot, 1969). The absence of borings within the cemented horizons could be explained by a lack of intense cementation leading to a 'soft' hardground, or by quick deposition of sediment burying the cemented crust and thus preventing bioerosion, as in the Persian Gull" today (Shinn, 1969). The formation of widespread submarine hardgrounds, for which very low erosion and sedimentation rates coupled with good water circulation are required (Shinn, 1969), could have been facilitated by the presence of a subtidal bacterial mat (evidence of presence of microbial tubes and bundles) that increased the resistance of the bottom sediment to erosion and consequently the contact time between the grains, like in modern occurrences (Little Bahama Bank, Bathurst, 1967; Neumann et al., 1970; Scoffin, 1970; Hardie and Ginsburg, 1977; Bermuda, Gebelein, 1969). The fossil biota diversity is greatly increased if con> pared to the inner platform facies (Fig. 6); in pmrticular, dasycladacean algae (GryphoporelIa curvata) are important contributors to the fossil fauna. Nonetheless, this fossil assemblage in other Upper Triassic carbonate platforms has been interpreted as characteristic of a restricted lagoonal euvironment with elevated salinities and sluggish water circulation (Berra and Jadoul, 1996: Climaco et al., 1997). This clearly does not apply to the outer platform facies of the DP, where the occurrence of hardgrounds points towards a good circulation of seawater, by comparison with modern occurrences (Taft et al., 1968; Shinn, 1969; Taylor and Illing, 1969; De Groot, 1969; Shinn, 1983). Moreover, the interpretation of monospecific dasycladacean occurrences as controlled by strenuous hypersaline environmental conditions is questionable (Fltigel, 1985), substrate type (determined by the water regime on the platform) being the controlling factor. The origin of the 'zebra rock' structure has not been resolved yet. Fischer (1966), who first introduced the term, interpreted it as being caused by desiccation of mud layers
Mt. Prainaggiore (Carman Prealps, northeastern Italy), outer platform and margin facies of the Dolomia Principale, Middle-Upper Norian. Slabbed specimen showing the vertical occurrence of stacked hardgrounds (arrows); note a large upturned lithoclast (L) creating a cm-wide shelter pore infilled by cements: scale bar is 1 cm. Thin section photomicrograph of subparallel sheet cracks in 'zebra rock' structure; each layer is floored by a dark mudstone (A) which is overlain by peloidal wackestone (B): the top of the layers are growth sites of radiating fibrous cements that seem to push apart the once adjacent walls of the cavity: scale bar is 500 btm. Thin section photomicrograph of contorted and undulated laminae with abundant microbialites (arrows); note the circular bioturbation cavities (B) concentrated around the microbial bundles and laminae; scale bar is 1000 gila. Polished slab of a digitate stromatolite mound where it is possible to recognize the upward welding of the single colunms to give a ~ structure; note also the intermound cements (small arrow) occurring together with geopetal infillings (darker patches in between the columns): scale bar is 1 cm. Polished slab of serpulid-microbial mound fragments redeposited within the bioclastic-intraclastic packstonegrainstone lithofacies; scale bar is I cm.
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within neptunian dikes. A structure termed "zebra limestone' was interpreted by Ross et al. (1975) as influenced by layered cyanobacterial mats that upon decay would leave a thin laminar cavity where precipitation of fibrous marine cements occurred creating the zebra limestone. Recently, Satterley (1994) interpreted 'zebra rocks' from the Rhaetian Dachstein limestone as a kind of stromatolite. In the case of the DP the positive relief of the structure (mound shape), the presence of filament moulds and also the lateral continuity of the laminae point towards a partial influence by microbial mats. The presence of a microbial mat, continuous and of a certain minimal thickness, could generate after organic matter decay subplanar cavities (as suggested by Ross et al., 1975). These cavities would have been partially filled by the peloidal mudstone and then by the marine fibrous cements, that, by growing away from both the floor and the roof of the cavities, enlarged them. Absence of any evidence for subaerial exposure rules out the possibility that the sheet cracks were originated in an inter-supratidal setting by dessication.
3.3 Margin facies This facies belt occupies the platform edge of the DP, partially extending downslope for a distance of a few meters. While the transition with the adjacent outer platform facies is rather sharp, the change into the upper slope facies is more continuous. The margin facies is composed of three lithofacies: 1) stromatolite mounds, 2) serpulid-microbial boundstone and 3) bioclastic-lithoclastic packstone-grainstone. The stromatolite mounds found within the margin facies can be subdivided into three categories based on their morphology and internal structure: (1) meter scale L L H - C and SH-C stromatolite mounds; (2) dm-tall digitate ones and (3) cm- to din-scale columnar strt, ctures. The former consist of mounds as tall as 5 meters (LLH-C - SHC, Logan et al., 1964) an vertically stacked to as much as 50 m, forming an elongated ridge that is parallel to the DP platform edge orientation. The average size of the mounds is roughly 1.5 m in height and 60-80 cm of lateral extent,
Plate Fig. 1.
Fig. 2. Fig. 3. Fig. 4. Fig. 5.
Fig. 6.
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with an estimated synoptic relief of 50 to 100 cm. The internal fabric of these mounds is made of laminae varying in thickness from 0.1-2 mm that pinch out laterally on acm scale. The growth bands are punctuated by an organic-rich rim and the sediment in between two organic-rich laminae is made of peloids and bioclasts. Fragments (20-50 cm in size) of the rigid mounds are found around the stromatolites. The interdomal sediments are made of bioclasticintraclastic packstones-grainstones in which the skeletal grains are echinoid fragments, small bivalves, fragments of dasycladacean algae and serpulid tubes, while the nonskeletal grains are sand-size peloids. The meter scale mounds are seen to initiate as planar to wavy laminae, in some cases broken into era-size fiat pebbles, followed by a vertical growth phase that produces the domal structure. A second type of stromatolite, making only a minor contribution to the bulk of the margin sediments, is digitate in shape, 1-3 cm wide and several cm long, with a layering style similar to that of the meter-scale domes, and is associated with peloidal-bioclastic wackestones-mudstones. On the outer surface of the stromatolites are microbialitic encrustations, ram- or cm-thick, made up of laminated micrite carrying serpulid tubes (P1.28/2). A third type of stromatolite consists of laminated mounds or columns that nucleate preferentially on breccia clasts made of bioclastic packstone-grainstone (lithofacies 3). They are few tens of centimeters tall and attain a tabular to vaguely mounded shape, with colmnnar forms coalescing towards the top ('cauliflower' structure) (PI. 27/4). The internal layering is generally well preserved and is made of regularly alternating organic- and sediment-rich laminae, pinching out sharply at the edges of the bioconstruction (PI. 28/1 ). The open spaces in between the stromatolite columns are filled either by a rim ofisopachous marine cement (PI. 27/4) or by bioclastic wackestonesmndstones. Serpulid-microbial boundstone (lithofacies 2) is characteristic of the upper slope facies but also occurs sporadically in the margin facies of the DP (see next section for a
Mt. Pramaggiore (Carnian Prealps, northeastern Italy), margin facies of the Dolomia Principale and upper slope facies of the Fomi Dolomite, Middle-Upper Norian. Thin section photomicrograph of a stromatolite mound nucleating on breccia clasts; note the good preservation of the carbonate layers outlined by organic-rich seams; the carbonate layers pinch out abrnptly at the edge of the stromatolite mound; scale bar is 750 tam. Thin section photomicrograph of digitate and columnar era-scale stromatolites; the stromatolites (1), which have been recrystallized, are encrusted by microbialites (2) and serpulids (arrow); scale bar is 750 btm. Upper slope dolo packstones-grainstones onlappmg steeply inclined breccias (black anow) and amalgamation of two breccia beds (white arrow); field of view is 100 m wide. Close up view of grain-supported breccias with slope clasts made of poorly consolidated dolo packstonesgrainstones (dark color); lens cap is 5 cm across. Polished slab of serpulid mound located on the upper slope; note the initial serpulid growth parallel to the outer surface of the underlying colony (arrow) followed by a vertical preferential orientation of the worm tubes; note also the paucity of intramound sediment; scale bar is 1 cm. Thin section photomicrograph of a serpulid colony; note the ahnost complete absence of microbial encrustations and a fi'amework cavity partially filled with peloids (dark) overlain first by an isopachous cement (1) and then by sparry calcite (2); scale bar is 750 lure.
Plate
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detailed description). Serpulids are preserved in their vertical growth position, either as isolated tubes or as small colonies, with a central bundle of tubes enveloped by a micritic microbialitic encrustation (few mm- to cm-thick). The encrusted colonies attained a certain degree of rigidity, also aided by the precipitation of early isopachous marine cements, making them prone to breakage, generating cm-size fragments redeposited in the bioclastic-intraclastic packstone-grainstone lithofacies (P1.27/5). Bioclastic-lithoclastic packstone-grainstone (lithofacies 3) makes a substantial contribution to the bulk of the margin facies, occurring as planm" laminated beds separated by sharply erosive undulatory surfaces. The grains (peloids and lithoclasts) are mainly coarse silt to medium sand in size and are frequently surrounded by a thin micritic envelope, closely resembling the one described in the outer platform facies hardgrounds. Both the skeletal and non-skeletal grains can also be encrusted by microbialites, causing a poor sorting and the preservation of articulated bivalve shells. The framework pores can be either occluded by geopetal sediments or by isopachous marine cements. Finally, fragments of Gr3,1~hoporella cun,ata can occur in deposits more than 1 meter thick, associated mainly with small bivalves and sand-size peloids, in the proximity of the meter-scale stromatolite mounds. The marine cements occur as isopachous fibrous rims, and when not recrystallized show the characteristic acicular shape and square tip terminations of aragonite, described in many other Triassic carbonate platforms deposits (i.e. Goldhammer, 1987; Berra and Jadoul, 1996). 3.3.1
Depositional Environment Interpretation
The three lithofacies encountered in the margin facies can all be confidently attributed to a subtidal bioconstructed platform margin. The m-scale stromatolite mounds and the serpulid-microbial boundstones constitute a remarkably different reel-building community from the well known coraland sponge-dominated Norian-Rhaetian counterparts worldwide (Fagerstrom, 1987; Stanley, 1988). Plate Fig. 1.
Fig. 2. Fig. 3. Fig. 4.
Fig. 5. Fig. 6.
29
The DP stromatolite mounds closely resemble in external morphology, style of layering and mode of nucleation the modern Bahamian subtidal mounds (Dravis, 1983; Dill et al., 1986; Dill, 1991 ; Browne, 1993). Similar stromatolite mounds have also been described in coeval DP margin facies in Lombardy (Jadoul, 1986: Cirilli and Tannoia, 1988; Berra and Jadoul, 1996) and in the Southern Apennines (Cirilli, 1993; Iannace and Zamparelli, 1996; Climaco et al., 1997) (see also Zamparelli et al., 1999, for a complete review). The small digitate forms are isolated occurrences and, together with the columnar forms, are always found facing the slope break of the DP platform, suggesting that they grew in a higher energy depositional environment than that of the meter scale mounds, explaining the inability to coalesce and form bigger mounds (Hoffman, 1976). Serpulid-microbial boundstones are associated with the decimeter-tall stromatolites nucleating on breccias and hardground surfaces. Although reinforced by microbial encrustations and by precipitation of early marine cements, they were prone to erosion by bioeroders (gastropods, echinoids) and by currents and storm-waves. Similar mounds at a DP platform edge have been reported from the margin facies in Lombardy (Jadoul, 1986; Cirilli and Tannoia, 1988; Jadoul et al., 1992, 1994; Trombetta and Claps, 1995; Claps et al., 1996; Berra and Jadoul, 1996) and in the Southern Apennines (Cirilli, 1993; Iannace and Zamparelli, 1996; Climaco et al., 1997). They m'e also present in the Upper Triassic Alpujfirride Complex of southern Spain (Fltigel et al., 1984; Martin and Braga, 1987: Braga and Lopez-Lopez, 1989). The occurrence of relatively large and heavily calcified dasycladacean algae within the bioclastic-lithoclastic packstones-grainstones, associated with subtidal hardgrounds, indicates an environment where large masses of marine supersaturated water passed through the upper centimeters of the bottom sediment gradually cementing the grains. Abundant dasycladacean deposits within the DP margin facies in Lombardy have been reported by Cirilli and Tannoia (1988), Jadoul (1986) and Trombetta and Claps (1995).
Mt. Pramaggiore (Carnian Prealps, northeastern Italy), upper and lower slope facies of the Forni Dolomite, Middle-Upper Norian. Ta-Tb Bouma sequence in the dolo packstone-grainstone lithofacies; note the basal sharp erosive surface (arrows), the graded interval (Ta) and the sharp transition to the laminated interval (Tb); magnifier is 1 cm across. Polished slab of upper slope dolo packstones-grainstones showing reverse grading at the base followed by a normally graded bed; scale bar is 1 cm. Channeled lower slope breccia; note the erosive scar at the base of the bed and the platform (white) and slope (dark) derived clasts; magnifier is 1 cm across. Polished slab of a thick dolo packstone-grainstone bed; note the richness in bioclasts (white elongated grains) and the l)resence of two large dark clasts which have been eroded from the underlying laminated bed; the poorly defined outline of the clasts suggests a low degree of consolidation attained by the laminated dolo wackestones-grainstones prior to erosion; scale bat" is 1 cm. Polished slab of laminated and graded dolo packstones-grainstones; note the sharp supmimposition of thinly organic-rich laminated units (1) over the graded beds (arrow); scale bar is 1 cm. Polished slab of graded dolo packstones-grainstones; note the sharp erosive lower boundaries of the graded beds (upward arrows) and the taint lamination at the top of each bed; scale bar is 1 cm.
Plate
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Go'phoporella curvata in the Upper Triassic of the Northern Calcareous Alps is found within the highest water energy facies in the reef complex (Senowbari-Daryan and Schfifer, 1979). The invertebrate non-reef building fauna of the I)P margin facies does ,lot differ substantially fi'om the outer platform facies of the DP; the only additions to the margin facies fossil biota are the stromatolites and serpulids (Fig. 6). Cementation was a common process within the margin depositional environment, given the ubiquitous occurrence of micritic and fibrous cements. Although these two types of cements may occur both in marine phreatic and marine vadose settings, given the absence of any feature indicative of the latter, we can conclude that the margin of the DP was constantly submerged and never exposed to subaerial diagenesis. 3.4 Upper slope facies Within the upper slope facies four different lithofacies have been distinguished: 1) megabreccias, 2) polygenic breccias, 3) bioclastic-intraclastic dolo packstone-grainstone, 4) serpulid-microbial mounds. Megabreccias (lithofacies 1) are localized adjacent to the platform margin and exclusively occur in tile stratigraphically youngest part of the upper slope complex (Pl. 28/3). The megabreccias are chaotic deposits composed of unsorted clasts, both angular and rounded in shape and ranging in size from tens of cm to 10 m. lu some cases the clasts occur as isolated boulders iuterbedded with dolo packstones-grainstones that are syndepositionally deformed into slumps and folds. The lithology of the clasts is primarily representative of the margin facies (serpulid-microbial mounds and small digitate stromatolite lithologies), with minor contributions from the upper slope. The megabreccias can be either grain- or matrix-supported. In the first case bioclastic-intraclastic packstones-grainstones are the 'matrix' of the deposit, while in matrix-supported megabreccias the clasts are embedded in intraclastic mudstones-packstones. No cementation occurred within and in between the breccia blocks. Polygenic breccias (lithofacies 2) are found throughout the slope, but are most abundant in the upper slope facies. They occur as lens-shaped accumulations 0.5-10 m thick; they pinch out downslope over less than a 10 m distance, are crudely normally graded, with some reverse grading at the base, and can also be amalgamated (PI. 28/ 3). The breccias typically have an erosive lower boundary with the underlying graded dolo pack-grainstones, and can be either grain- or mud-supported. In the first case the intergranular material is made of a bioclastic grainstonepackstone in which silt- to sand-sized grains (peloids and irregularly-shaped mudstone intraclasts) are dominant, with a skeletal fraction made ofechinoids and thin shelled bivalves. Mud-supported breccias are characterized by a mudstone matrix in which the clasts are embedded. The bulk of the polygenic breccias is made of clasts varying in size from a few centimeters to almost I m that come mainly from the margin (serpulid and stromatolite clasts) and the
upper slope facies (bioclastic-intraclastic dolo packstonegrainstone clasts, see below) (PI. 28/4). Bioclastic-intraclastic dolo packstones-grainstones (litbofacies 3) are found throughout the slope but increase in abundance downslope. They occur in thin beds (2-3 crn to 10 cm thick on average) that typically overlap and onlap both breccia beds and serpulid-microbial mounds (P1.28/ 3), the bases of the beds being usually sharp and erosive. The beds show a regular internal organization of the sedimentary structures, from bottom to top, as follows (P1. 29/1): (l) an erosive basal surface, (2) a normally graded interval and (3) a laminated interval. The laminated cap lies on the graded unit with a sharp transition; it is made of alternations of white clastic laminae, usually 1-3 mm thick, composed mainly of small (0.5-1 mm) lithoclasts, and organic-rich mudstone laminae with irregular lower and upper boundaries that show preferential compaction. 111 some cases, especially closer to tile margin facies, the laminated cap is lacking, and the beds consist simply of the normally graded interval. Commonly the dolo packstonesgrainstones are deformed into meter-scale slump folds, in some cases localized below massive breccia beds. In the upper part of the upper slope reverse grading limited to few beds has also been observed (Pl. 29/2). Lithofacies 3 contains irregularly shaped and plastically deformed intraclastic mudstone grains (coarse to very coarse sand in size), rounded peloids (coarse silt-very fine sand size), oncoids and bioclasts (crinoids, gastropods, thin shelled and ornamented bivalves, brachiopods, rare benthic forams (Aulotortidae), problmnatica (Cayeuxia), serpulids, conodonts and rare rounded Parafavreina pellets). The serpulid-mic,'obial mounds (lithofacies 4) occur on the eastern side of Mt. Pramaggiore, within the stratigraphically older upper slope facies (see 5.1.1 ), attaining an individual thickness of 1-5 meters and nucleating on previously deposited breccias and dolo packstones-grainstones. Two different types of serpulid-microbial boundstones have been recognized. The first is similar to the one previously described in the margin facies, with abundant intramound early isopachous cement. The second type consists of a mass of serpulid tubes that makes up the bulk of the mound, with rare and thin micritic coatings. The vertical accretion of the serpulid mounds occurred in multiple phases, initially with the serpulid tubes aligned parallel to the outer surface of the underlying colony, and subsequently with growth at right angles to the surface (PI. 28/5). Associated with these mounds are small cm-tall finger stromatolites nucleated on serpulid colonies or pelecypod shells. In some cases they completely envelope pelecypods, brachiopods and small gastropods. The framework pores and cavities are preferentially filled with peloidal-bioclastic wackestones and early isopachous cements (Pl. 28/6). The intermound sediment is a bioclastic packstonegrainstone with abundant fragments of serpulid tubes and thin-shelled ornamented and elongated bivalves, probably indicating a semi-infaunal mode of life. Other bioclasts are crinoids and rare forams (Nodosaridae), and coarse silt- to fine sand-sized peloids are also present. The sediment grains
165
are surrounded by a thin micritic fihn, showing that the microbial binding and stabilizing activity was not confined to the mounds but extended into the adjacent sediment. Partial silicification of carbonate grains has produced nodules and lenses of dark fine-crystalline silica, possibly originated from diagenetic rimobilization from siliceous sponges, as suggested by Claps et al. (1996).
3.4.1
Depositional Environment Interpretation
The upper slope environment was characterized by pcriods of intense redeposition of margin material and reworking of slope clasts alternating with prolonged periods when no or little sediment movement was taking place, allowing the serpulid mounds and the digitate stromatolites to form and develop. The serpulid mound types, ranging IYom well developed 1-5 meter tall to stunted forms, presumably reflect different hydrodynamic conditions and degrees of environmental stress (high sedimentation and erosion rates). The meter scale serpulid mounds show a remarkable similarity, in the way of nucleating and growing, with the recent Baffin Bay (southern Texas) serpulid colonies ('random" and 'vertical' growths of Andrews, 1964; see also Cole. 1981). The serpulid-microbial mounds acted as sediment binders, stabilizing the breccia deposits on the slope, preferring a hard substrate for their nucleation and development with little or no mud fraction, and were easily brecciated and eroded, becoming in situ sediment sources. The sedimentary processes dominant on the upper slope depositional environment were essentially all driven by gravity (mass transport processes). Megabreccias probably originated from slides and rockfalls. The R)lds in the dole pack-grainstones, with their strikes being consistently aligned parallel to the slope profile, are interpreted as slumps, in some cases triggered by the deposition of the megabreccia beds. Both slumps and megabreccias could have been triggered by wave and storm physical erosion of the margin, fault scarps developing along the margin, earthquake shocks and continuous slope creep. The polygenic breccias are interpreted as debris l]ow deposits based on: 1) the erosive character of the bottom boundary, 2) the matrix-supported fabric, 3) the chaotic arrangement of the clasts, 4) a crude normal vertical grading, as well as an inverse one (rarer), 5) plastic d e f e r mation of the clasts entrained in the flow, 6) the wide range in clast sizes resulting in poor sorting, 7) the rapid lateral pinch out of the deposits, and 8) the associalion with turbulent flows deposited on top, producing low density turbidite sequences. Tongue-like breccias thinning later ally over a short distance are interpreted as the sudden freezing of debris flows. The sharp contact between brec cias and the overlying laminated and graded packstonesgrainstones is interpreted as the downslope evolution of debris flows into more diluted and turbulent l]ows (Krause and Oldershaw, 1979). The bioclastic-intraclastic dole packstones-grai nstones are interpreted as turbidites. The classic Bouma sequence (low-density turbidites, Bouma, 1962) never occurs, instead Ta-Tb intervals are the only ones present in the upper
slope facies, more characteristic of high-density carbonate lurbidites (Colacicchi and Baldanza, 1986: Eberli. 1991 ). These types of I]ows need steep angles to cover long distances in order to cmltrast the abundant loss of kinetic energy by grain-to-grain collisions, conditions that were met on the stccp angles of the upper stopc( 14-36 degrees) (see section on dm geometry of the slope). Inversely ~zradcd mud-free beds anti matrix-supported bioclastic-intraclastic dole packstoncs-grainstones point towards a support mechanism like grai n to grain collisions and matrix strength buoyancy (density-modified grain flows of Lowe, 1976a). rather than iluid turbulence. This is in agreement with the overall expected abundance of high-density turbidity flows upslopc and low-density ones downslope (I,owe, 1982; Piper and Stow. 1991 ). The following mechanisms could have actcd either simultaneously or alone on the upper slope deposits causing sediment l iquc faction and successive downslope movement of gravity flows: I) earthquake shocks, 2) rapid consolidation of looscly packed sediments by sudden loading associated to deposition of overlying beds, 3) failure of loosely-packed sediments close to their angle of repose, 4) loss of support due to erosion or sediment creep, and 5) failure along discrete planes of weakness such as mud laminations or layers of organic debris. The slope and basinal deposits of the I)F are rich in organic matter and likely were loosely packed due to rapid rcdeposition of the grains, so that the conditions for sediment liquefaction could [lave been easily reel, Some of the laminated dole pac kstones-grainslones show a very sharp boltom boundary with the underlying graded beds. a relationship that leads to interprel them cithcr as the Tb Bouma interval, or as traction load deposits laid down by very diluted llows (silty turbidites of Stow and Shanmugan, 1980: Piper and Stow, 1991), or else contourites. Little remobilization el' sediment, possibly due to normal fair weather wave movement on thc bottom sediments, would be capable of initiating low-density silty' mrbiditcs. Muddy and sandy contourites (Stow and Lovell, 1979) could have produced the latnination style obser,~ed on the upper slope, although it is di t'ficult to imagine how contour currents could be generated in such a shallow basin (301)-400 m deep) that likcly lacked a thermo-hatine circulation. All considered, the depositional scenario for the upper slope lacies was characterized by maior catastrophic events, when the breccias and the megabreccias deposited, alternating with periods of relative stab ity when Ta-Tb turbidiles, high- and h)w-dcnsity turbidites deposited.
3.5 Lower slope facies The DE lower slope deposits are composed of two lithofacies: I) laminated anti graded dole packstonesgrainslones and 2) lithoclastic breccias. Tile transition with the upper slope facies is gradual and arbitrarily placed in the field where polygenic breccia deposits become rarer and the laminated and graded dole packstones-grainstones incrcasc in abundance. LithoI:acJes 1 consist of alternating beds of laminated
166
and graded dolo packstoncs-grainstones. The laminated beds consist of an alternation of white or light grey 0.1-2 mm thick laminae, with grain-supported fabric made of peloids and silt-sized bioclasts, and dark grey-black 0.1-2 mm thick dolo siltstone-mudstone laminae rich in organic matter (see P1.29/5) with common organic seams. Graded dolo packstones-grainstones have a bed thickness between 1-10 cm (see P1. 29/6). Normal vertical grading is most common, but reverse grading can occur, in some cases the result of amalgamation of two normally graded beds. The contact between the graded and laminated units is typically quite sharp, in few cases gradual. The thickest beds contain plastically deformed upper slopederived dolo packstones-wackestones clasts, pointing to a semi- or unconsolidated physical state at time of erosion, giving the rock a flaser breccia structure (Davies, 1977) (Pl. 29/4). The graded beds are bioclastic-intraclastic wackestones and grainstones with crinoid fragments, small bivalves, peloids, oncoids, large thin shelled gastropods and sparse phosphatic remains. Matrix can vary significantly within one single graded bed, and usually its fraction increases as the grain size decreases. The sorting is usually poor, with a range in the grain sizes from medium silt to coarse sand. Where the laminated and graded dolo packstones-grainstones are overlain by a thick debris flow deposit, they have deformed into centimeter- to decimeterscale slump folds. Lithoclastic breccias (lithofacies 2) have angular to rounded clasts from few cm up to 2 m in diameter, reflecting a both margin and slope origin. The breccias exhibit a very poor sorting, a sharp, erosive and sometimes channeled base (PI. 29/3), an upper surface varying from ragged to smooth with a frequent vertical transition into the laminated lithofacies 1. Both mud- and grain-supported breccias are present, where the grains of the matrix are bioclasts (bivalves and crinoids), peloids and irregularly-shaped dolo mudstonesgrainstones ctasts. 3.5.1
Depositional Environment Interpretation
The mechanisms that seem to be responsible for carbonate deposition in the lower slope are debris flows, highand low-density turbidites, possibly contourites and mud settling from suspension. Breccias, as discussed in the upper slope section, can be interpreted as deposited by debris flows. Laminated dolo packstones-gainstones can be interpreted either as distal low-density turbidites or as contourites (see discussion in the previous section). The percentage of laminated dolo packstones-grainstones in the lower slope facies is far higher than in the upper slope deposits, indicating the predominance of low-density turbidity flows in the lower part of the slope. The alternation of silty ram-thick laminae and organic-rich mudstones can be explained also by the alternation of episodic sedimentation via distal turbidites and more 'constant' settling of sediment from suspension (Stow and Bowen, 1980). The laminated silt-mud alternations fit in the T3 interval of the classification of finegrained turbidites by Stow and Shanmugan (1980).
The graded beds of lithofacies 1 can be interpreted as high-density turbidites. Their shaq~ boundary with the overlying laminated dolo packstones-grainstones suggests that either the laminated interval was part of the same turbidity flow separated from the lower high density current, or was deposited as a reworked unit by a low-density turbidity current or by contourites. Flaser breccias are very similar to the ones described by Schlager and Schlager (1973), where plastically deformed silty-muddy laminae are entrained in the flow of a high-density turbidite as it scoured into the underlying laminated lithofacies 1. The 1-3 cm thick units showing normal grading in a fine sand to coarse silt matrix, are interpreted low-density (distal) turbidites (Walker, t967; Schlager and Schlager, 1973; Davies, 1977). 3.6
Basinal facies
The basinal facies of the DF are only partially represented in the study area. They outcrop more continuously to the east ofMt. Rua (see Fig. 4), where the transition from the lower slope to the basinal facies is marked by a gradual thinning of the beds and decrease in sediment size. This is particularly marked in the breccias, which become progressively fewer and thinner, with clasts usually not more than 1-2 cm across. Breccias make up only a small fraction of the basinal facies, which are mainly made of two lithofacies: 1) laminated and graded dolo packstonesgrainstones and 2) homogeneous mudstones. Laminated and graded dolo packstones-grainstones are identical to the ones of the lower slope facies. The laminated beds are more common in the basinal facies, while the graded beds become less and less abundant. The overall thickness of the graded beds is 2-5 cm on average, while the dark-light laminae maintain the same thickness as in the lower slope facies. The packstones and grainstones decrease in grain size from medium-coarse sand to fine sand and silt, while the centimeter mudstone clasts become rarer. As a result, on average, grain-supported fabrics decrease and the matrix-supported ones become predominant. Bioclasts are mainly bivalve shells and crinoid fragments. Evidence for the presence of bottom epifauna or infauna is lacking. Homogeneous mudstones (lithofacies 2), 0.5-3 cm thick on average, become increasingly more abundant towards the deeper part of the DF basin. They have a homogeneous appearance, are rich in organic matter which is concentrated in horizontal seams as the result of compaction, and are frequently eroded. 3.6.1
Depositional Environment Interpretation
The basinal facies of the DF in the study area is more representative of the basin margin than the basin center. Graded and laminated dolo packstones-grainstones can be interpreted as distal turbidites (see discussion in the upper slope section). As the general fining outward trend occurs, the graded beds become rarer while the silty laminae become the predominant sedimentary facies.
167
Fig. 5. Physical processes and early diagenesis affecting the scdimenlary facies ollhe 1)P. For exact lateral facies widths refer lo Fig. 9. and for paleo water depth refer to values in Tab. 2. The mudstone layers make a first significant appearance in the basinal facies. Given the absence in the [,ate Triassic of calcareous nannoplankton, they can be either interpreted as the result of settling from the cloud of suspended sediments above a turbidity current (flow lofting of Sparks et al., 1993; Reading, 1996) or as gravitational settling from sediment-laden surface waters originated by storms and tides on the shallow-water carbonate bank and then exported offshore by winds and tides, as occurs today in the Bahamas (Heath and Mullins, 1984: Mullins et al., 1984). The preservation of the mudstone layers as distinct units, as well as the mm-lamination of the dolo packstones-grainstones, is due to the absence of bioturbators living on the DF sea floor. The anoxic bottom conditions favored the preservation of an allochtonous rich flora and fauna (Dalla Vecch ia, 1990), in conjunction with high sedimentation rates rapidly burying the organic remains.
4 THE DOLOMIA PRINCIPALE DEPOSITIONAI, MODEL
The depositional and early diagenetic processes affecting the DP platform in the Mt. Pramaggiorc area arc summarized in Fig. 5, while the fossil fauna abundance is shown in Fig. 6. The Dolomia Principale in the study area can be classified as a storm-dominated shallow-water carbonate platform. The physiography of the platform was characterized by tidal flats in its interior parts. This protected depositional environment, affected only by storms and significant tides and wind-driven waves, was inhabited by a restricted
fauna due to the long residence time of seawater on the shallow bank anti the consequent tendency to salinity fluctuations. As suggested bv the presence of algal-bactevial tufa, inhospitable physico-cheinical conditions on the high inlertidal-supralidal parts of the depositional systems were made even more extreme duc Ihe seasonal freshwater input. Seaward fvom the tidal Ilals, a shallow water lagoon with a more diverse open-marine fauna existed, vellection of the improved seawater circulation. Storms where responsible for intense winnowing of the fine fractions that were transported bolh landward (storm layers m the inter-supratidal laminites of the in ner platform) and seaward (Fig. 5 ), leaving behind the sand- and silt-size particles as lag deposits that were cemented to form submarine hardgrounds, hnportant in reducing the vale of erosion in the DP ouler platlorm was the binding and trapping by bacterial mats of the bottoln sedimcnls. Coavse grained bioclastic inputs into the outer plattk~vm areas closest to the margin provide evidence of the effects oFboth physical and biological erosion ol'the margin mounds, acting as a m~tior source of debris for the a({jacent platform deposits. The DP platform margin was mosl ly bioconstvucted by meter-scale stromalolitc mounds and subordinalely serpulid-microbial mounds. Even though serpulid-microbial mounds are common in the margin facies, thcy never seem to have reached the predominant role recognized in the coeval l,ombardy and Apennines DP margins (see Zamparelli el al., 1999, for a review). The absence ofsubaqueous sand dunes and oolites suggests weak tidal currents affecting the DP margin sediments. Although storm waves must have impinged onto the platfovm margin, ihe presence of stromatolite and serpulids rnounds, combined with bottom
168
Fig. 6. Distribution of fossil in situ biota across the platform-to-basin transition in the Mt. Pramaggiore area. Data are qualitative. For exact lateral facies widths see Fig. 9. sediment binding by bacterial mats, prevented the formation of large scale bedforms, dissipating the wave energy. Early syndepositional marine cementation (micritic and fibrous cements), the result of good water circulation through the margin of the DP platform, together with a high microbial trapping and binding activity, was an important process that allowed the stabilization of the bottom sediment and gave strength to the serpulid-microbial mounds. This process also provided a hard substrate for both stromatolite and serpulid-microbial mounds to nucleate, indispensable for the latter to successfully initiate the bottom sediment colonization (Andrews, 1964; Bosence, 1979; Cole, 1981 ). Early diagenesis throughout the DP platform depositional environments was almost entirely marine phreatic (micritic and isopachous fibrous cements), with the exception of the inter-supratidal flats where both marine and freshwater vadose conditions existed.
4.1
Middle Norian climate and fossil biota of the DP margin facies
DP margin facies similar to the ones at Mt. Pramaggiore have been reported from other parts of the Southern Alps (Jadoul, 1986; Cirilli and Tannoia, 1988; Jadoul et al., 1992, 1994; Trombetta and Claps, 1995; Claps et al., 1996; Berra and Jadoul, 1996) and Southern Apennines (Cirilli, 1993; Iannace and Zamparelli; 1996, Climaco et al., 1997) (see Zamparelli et al., 1999 for a complete review).
Bosence (1973, 1979), ten Hove (1979) and ten Hove and van der Hurk (1993) stressed the fact that serpulids are opportunistic animals that can survive in a wide range of habitats. In the absence of competitors for space and food they will become the dominant species in the environment. A similar consideration can be made for the development of microbialites, as pointed out by Garret (1970). The exclusion from the Norian shallow-water carbonate platform margins of the well known ScleractinianSphinctozoan reef building fossil community in the Southern Alps and Apennines has been explained by Zamparelli et al. (1999) by: 1) low oxygen concentration in sea water and 2) anomalous salinity (mesosalinity). While point 1 would apply to the upper slope serpulid-microbial mounds, clearly it does not fit the DP margin depositional paleoenvironment at Mt. Pramaggiore which was located in normally oxygenated shallow-water and where water circulation was storm-dominated. Anomalous salinity values of seawater due to intense evaporation under an arid climate are not supported by the sedimentary facies and open marine fossil biota of the DP margin. The inner platform deposits, as discussed before, resemble closely the modern deposits of tropical humid tidal flats (Bahamas), lack evaporite minerals and contain evidence for seasonal freshwater inputs, confirmed by similar findings in the coeval Hauptdolomit of the Northern Calcareous Alps (Fruth and Scherreiks, 1982, 1984). A seasonal humid tropical climate for the Middle Norian can be inferred also from the results of a preliminary palynological study on the organic-rich beds within
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Fig. 7. Schematic representation of the two possible scenarios occurring at the edge of lhc I)P platform during a relali,,e sealevel high (Case 1) and a relative sealevel low (Case 2). See text for description of the two models. the DP inner platform deposits (Cozzi and J~ger, 2000) which, with their continental-dominated OM, points towards a climate capable of sustaining abundant flora, with at least a seasonal freshwater input. Zankl (1971) reached the same conclusion to explain the abundant coal-like deposits and organic-rich horizons in the Hauptdolomit of the NCA. Moreover, famous freshwater fish remains (Gorjanovic-Kramberger, 1905) and characeans (Zankl and Merz, 1994) have been reported from the Steinplatte area. In addition, Zankl and Merz (1994) suggest a [reshwater origin for the 'ultra backreef' facies of the Hauptdolomit of the NCA, hypothesizing a situation similar to the modern Florida Everglades (Merz, 1992). A tropical monsoonal climate, combined with low amplitude-high frequency sea-level oscillations and with the physiography of the DP at Mt. Pramaggiore can explain the peculiar biota assemblage of the margin facies of the DP (Fig. 7). Two cases are considered: Case I shows the circulation pattern on the platform during a relative sea-level high and Case 2 during a sea-level low. In the first case a good circulation on the platform top is eslablished with export of sediment off-bank as a result of storm erosion of the bottom sediment in the outer platform facies. This fact would harm the potential growth of coral corn munities because of the high suspended sediment in the
waters moving off-shore, enhanced by the lack el'an active barrier like sand-waves shielding tile margin communities from the sediment-laden waters, as on today's leeward side of Andros Island tHine and Neumann, 1977: Hine et al., 1981a, by. In addition, evaporation of the bank-top waters would lead to an increase in tile salinity, and as these sahicr waters move offshore, in the absence of physiographic barriers, tile}; would damage the coral comrnunilies which are intolerant towards changes in water salinity. In Case I tile seasonal humid climate would generate a seasonal fresh;,~ater lens on tile DP tidal fiats, but because of lhe relative high position of sea-level it would not reach a great depth before getting mixed with the marine wateis. During a relative sea-level low (Case 2. Fig. 7) the active transport of sediment off-bank would probably be shtll off by lhe sluggish circulation on the shallow bank top. At the same time. during the humid season, the tidal fiats (which from field evidence reached a mininaunl dislance of only 500 m from the DP margin) would develop a much deeper freshwater lens. Due to the shallowness of tile water on the bank and the sluggish circulation, the salinity' of(he waters adjacent to the flats would be changed during intense runoff and lhrougll groundwater mixing (Berra and Jadoul, 1996). Buoyant density-driven water
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Average
Outer platform field-measured bedding dips
Upper slope field-measured bedding dips
Lower slope field-measured bedding dips
Basin field-measured bedding dips
22, 25, 25, 30
30, 30, 30, 32, 32, 35, 38, 38, 38, 43, 43, 45, 45", 45", 46", 53*
20, 40*
10
25.5
37.5
20
10
15-25
5-15
1-3
Depositional angles 0.5-1 (calculated and averaged)
Upper slope depositional angles (stage 1)
Upper slope depositional angles (stage 2)
11, 15, 17, 18, 19, 19, 22, 24, 24
21, 25, 26, 32, 36
Average
18.7
27.6
Std. deviation
4.23
5.39
Table 1. Summary of all the bedding dip data used in the calculation of the original depositional slope angles. See text for description of procedure and results. circulation could be established during the rainy season beneath the freshwater lens within the mixing zone (Whitaker and Smart, 1993; Whitaker et al., 1994; Vahrenkamp and Swart, 1994), this providing the mechanism for the movement of brackish mixing-zone waters from the edge of the tidal flats towards the outer platform and margin. The seasonal salinity fluctuation would harm the scleractinian
corals, which today are stenohaline (salinity of 34-36 %0) and prefer low water turbidity (Fagerstrom, 1987). The stress on the reef community would also be a function of the duration of the inimical physico-chemical conditions. Therefore, it is possible to explain the presence of the serpulid-microbialites in the Norian reefs taking into consideration local inhibiting factors, such as: 1) physico-chemical
Sedimentary facies
Stage 1 (Early-Middle Alaunian) calculated water depth
Stage 2 (Late Alaunian-Sevatian?) calculated water depth
Outer platform
0-13 m
0-13 m
Margin
13-20 m
13-20 m
Upper slope
20-190 m
20-280 m
Lower slope
190-265 m
280-360 m
Basin
280 m
380 m
Table 2. Predicted water depths across the platform-to-basin transition at Mt. Pramaggiore using the data shown in Table 2. Note the increase in water depth from Stage 1 to Stage 2.
Fig. 8. Simplified topographic map of the Mt. Pramaggiore area. The values of the upper slope depositional angles, corrected for the post-depositional tilting, have been plotted (grey area). Note the increase in the depositional slope angles from South to North (arrow), following the stratigraphic up and the change from DP pro~adation (Stage 1, Early-Middle Alaunian) to vertical aggradation (Stage 2, Late Alaunian-Sevatian?).
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Fig. 9. Schematic representation of the geometric profile of the platform-to-basin transition at Mr. Pramaggiore. Note lhe increase in the platform relief from Stage 1 to Stage 2, concomitant with a switch from lateral progradation to vertical aggradation of the DP platform. For facies water depth refer to values in Tab. 2. condition of the waters (salinity and sediment-suspension load), 2) the physiography of the platform margin (absence of a physiographic barrier such as islands or sand waves). and 3) relative sea-level position.
THE FORNI DOLOMITE DEPOSITIONAL MODEl,: R E C O N S T R U C T I O N OF THE ORIGINAl, SLOPE GEOMETRY
Along the Mt. Pramaggiore transect the original depositional angle of the slope sediments has been calculated. The field data (listed in Table 1) were corrected first lor Alpine tilting of the beds, via subtracting the average dip value of the outer platform facies (25 degrees) from all the facies belts by means of a rotation along the strike axis of the outer platform facies (NI20E), and then adjusted for the apparent dips using the table of Billings (1972). The final results are shown at the bottom of Table I and, in
more detail, on the map of Fig. 8. where tile true dip values are located exactly along the slope. The oulcr platform facies have becn considered, aJ'ter the till correction and also by comparison with modern carbonate bank tops, to have a very small dcpositional angle, in the order o1"0.5-1 degrees. Given the impossibility of determining the dip of the margin facies because the massi vc appearance and lack of well bcddcd units, an arbitrarily value o l 4 degrees has been chosen for its true dip. Subsequently. using the technique of Satlerley (1994), known tbc values of( 1) the lateral extent of each facies and (2) the depositional slope angle, the water depths corresponding to every single facies were calculated (Tablc 2). The formula used is: water depth tan (slope angle)
=
facies lateral extent
In calculating the water depth the slope angles have been distributed evenly within each single facies, i.e. three
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Fig. 10. Sketch illustrating the sedimentological changes in both platform and slope deposits during Stage 1 and Stage 2. See text for description. values (the average one _+ the standard deviation) have been used for equal amounts of lateral distance (Fig. 9), which have the effect of making the final profile of the transect look planar and concave upward, similar to those of modern carbonate slopes (Schlager and Camber, 1986). Adams et al. (1998) interpreted such profiles as characteristic of strong prograding or vertically aggrading depositional systems, where gravity-driven slope transport processes are dominant over sea-level fluctuations.
5.1 Evolution of the geometry of the slope The map of Fig. 8 shows a pattern of change in the dip angle of the upper slope facies, from values ranging between 11 and 24 degrees on the south-eastern side of Mr. Pramaggiore, to slope angles that are 10 d e ~ e e s higher on average (21~ ~ just to the north of it (Fig. 8, Table 1). This trend is paralleled by three more changes: 1) the facies association in the second group of values shows a decreasing amount of upper slope serpulid-microbial mounds and the appearance of megabreccia beds (Fig. 10); 2) the first group of values correspond to a phase of lateral platform
progradation while the second one to a phase of vertical aggradation, and 3) the second group of values was measured in rocks stratigraphically overlying those from which the smaller slope angle values were obtained. Within this stratigraphically younger slope complex a rich conodont fauna has yielded Late Alaunian-Sevatian (?) ages (Cozzi and Podda, 1998, Cozzi, 1999). These pieces of evidence clearly point towards a steepening of the upper slope facies in time, together with a contemporaneous change in the basic sedimentological features of the slope deposits and by an increase in the platform relief, reflected in an increase of water depth in the basin (Fig. 9, Table 2). The change in slope characteristics and their significance are discussed below, with the Early-Middle Alaunian slope complex discussed first (Stage 1) followed by the Late Alaunian-Sevatian one (Stage 2). 5.1.1
Stage 1: Early-Middle Alaunian
The Early-Middle Alaunian slope complex has upper slope angles that range from 11 ~to 24 ~(Table 2). Serpulidmicrobial mounds associated with small stromatolite bio-
17 9,
Fig. I 1. Summary of the different synsedimentary disrtlption features recognized wilhin the Dolomia Principalc in the study area. They are arranged in a clockwise order following their increasing overall size. From Cozzi (2000).
herms are very common and represent a downslope extension of the margin facies of the DP during a lime of platform progradation. Laminated and graded dolo packstones-grainstones and breccias build the bulk of the slope deposits, while megabreccia units are absent (Fig. I 0). In Fig. 9 a complete profile from the flat bank top (outer platform facies) to the basin (basinal facies) shows that at a distance of almost 1.5 km from the DP margin the water depth would have been approximately 280 m. This slope profile was attained during a time of DP platform progradation, recorded not only in the Mr. Pramaggiore area but also in the entire Carnian Prealps. 5.1.2
Stage 2: Upper Alaunian-Sevatian
For Stage 2 the upper slope angles range from 21 ~ to 3&, 9 ~ 11 ~ more than the Stage 1 slope (Table 1 ). In addition. water depth at Mt. Rua was 380 m during Stage 2, 100 m deeper than during Stage 1 (Table 2, Fig. 9), a consequence of the increase in the platibrm relief via vertical aggradation. The steepening of the slope is accompanied by an increase
in the abundance of breccias, by the appearance of megabreccias and by generally coarser grained turbidites in the upper slope, while the lower slope and basinal facies are mainly unchanged wilh respecl to Stage 1. Upper slope mounds become much less common on the sleeper Stage 2 slope, probably due to increased slope instability and erosion rate (Fig. 10). 5.2 Discussion Of the many classifications oFcarbonate platlkwm slopes arisen in the last two decades, based on both modern and ancient examples (Wilson, 1975; Playford. 1980: Read, 1982, 1985; James and Mounljoy, 1983: McIlrealh and James. 1984: Mullins and Cook. 1986). the most appropriate for the DP platfornl to basin transition during stages I and 2 is the one proposed by Mullins and Cook (1986), the slope apron lnodel. This model predicts a gradual transition between the margin facies and the beginning of the slope, which is in agreemenl with the field data at Mt. Pramaggiore where no marginal escarpment is present anti no by-pass
174
Fig. 12. Areal distribution and orientation of the synsedimentary disruption features within the Dolomia Principale. Note the occurrence of the NE-SW oriented ones several kms away from the platform margin at Mt. Pramaggiore and their parallelism to the strike of the Dolomia Principale platform edge. Modified from Cozzi (2000) with new data from Peruz (1999). area is found in the upper slope. The sharp platform slope break, contained within the DP margin facies, is also characteristic of a depositional or accretionary margin of a rimmed shelf (Read, 1985), where again margin and upper slope facies interfinger rather than abutting against each other. The slope apron model predicts a gently inclined slope (
