Facies architecture of an isolated carbonate platform: tracing the ...

Sedimentology (1999) 46, 893±912

Facies architecture of an isolated carbonate platform: tracing the cycles of the LatemaÁr (Middle Triassic, northern Italy) È NSEL 2 , *, THILO BECHSTA È DT*, SVEN O. EGENHOFF 1 , *, ARNDT PETERHA È TSCH 3 , È HLKE* and JU È RGEN GRO RAINER ZU *Geologisch±PalaÈontologisches Institut der UniversitaÈt, INF 234, D-69120 Heidelberg, Germany (E-mail: [email protected]; [email protected]) Shell Research and Technical Services, Volmerlaan 8, 2288 GD Rijswijk, The Netherlands ABSTRACT

The 720-m-thick succession of the Middle Triassic LatemaÁr Massif (Dolomites, Italy) was used to reconstruct the lagoonal facies architecture of a small atoll-like carbonate platform. Facies analysis of the lagoonal sediments yields a bathymetric interpretation of the lateral facies variations, which re¯ect a syndepositional palaeorelief. Based on tracing of lagoonal ¯ooding surfaces, the metre-scale shallowing-upward cycles are interpreted to be of allocyclic origin. Short-term sea-level changes led to subaerial exposure of wide parts of the marginal zone, resulting in the development of a tepee belt of varying width. Occasional emergence of the entire lagoon produced lagoon-wide decimetrethick red exposure horizons. The supratidal tepee belt in the backreef area represented the zone of maximum elevation, which circumscribed the sub- to peritidal lagoonal interior during most of the platform's development. This tepee rim, the subtidal reef and a sub- to peritidal transition zone in between stabilized the platform margin. The asymmetric width of facies belts within individual metre-scale cycles was caused by redistribution processes that re¯ect palaeowinds and storm paths from the present-day south and west. The overall succession shows stratigraphic changes on a scale of tens of metres from a basal subtidal unit, overlain by three tepee-rich intervals, separated by tepee-poor units composed of subtidal to peritidal facies. This stacking pattern re¯ects two third-order sequences during the late Anisian to early middle Ladinian. Keywords Carbonate platform, depositional palaeorelief, lateral facies

changes, supratidal tepee belt, windward/leeward effects.

INTRODUCTION In both siliciclastic and carbonate sediments, the documentation of lateral facies variation is critical for palaeoenvironmental and palaeotopoPresent addresses: 1Institut fuÈr Angewandte Geowissenschaften II, Technische UniversitaÈt Berlin, ACK 14, Ackerstraûe 71±76, D-13355 Berlin, Germany (E-mail: [email protected]). 2Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon SK, S7N 5E2, Canada (E-mail: [email protected]). 3Abu Dhabi Company for Onshore Oil Operations (ADCO), Geological Department, PO Box 270, Abu Dhabi, United Arab Emirates (E-mail: [email protected]). Ó 1999 International Association of Sedimentologists

graphical reconstruction. Accurate correlation is crucial to such reconstructions. In this paper, we show how we have taken advantage of excellent lateral and vertical exposures in order to document the detailed internal structure of the Middle Triassic LatemaÁr Massif, an isolated, atoll-like carbonate platform in the Dolomites of northern Italy (Fig. 1). Despite the dif®cult alpine topography, it is possible to trace time-equivalent units for several kilometres within the cyclic, lagoonal succession (Fig. 2). Continuous exposure, little tectonic disturbance and only localized dolomitization allow a detailed analysis of the carbonate build-up as a whole, and its individual shallowing-upward cycles (Goldhammer et al., 893

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1987, 1990). Four distinct marker horizons, which correspond to different stratigraphic levels of the build-up, were selected for the investigation of lateral facies variability (Fig. 3). These markers are characterized by lateral uniformity (e.g. red exposure surfaces) and are easily recognizable in the ®eld. Because of their distinctiveness, the marker horizons can be traced within different structural blocks and allow correlation over about 3 km. A minimum of 10 carbonate beds above and below each marker horizon was traced and correlated. The results presented here are based on the facies changes within these

N

Fig. 1. Localities of important Middle Triassic carbonate platforms in the Dolomites of northern Italy. In South Tyrol, with both German and Italian names. Magmatic centres: Predazzo and Monzoni (after Bosellini, 1991).

packages, and on the study of several sections covering the entire stratigraphic succession of the LatemaÁr build-up. GEOLOGICAL SETTING In late Anisian (early Middle Triassic) time, the Dolomites formed part of a continental shelf on the north-western rim of the Tethys ocean (Dercourt et al., 1993). In our study area, the shelf comprised the carbonate ramp of the Contrin Formation, characterized by deep marine

Fig. 2. Panorama of the LatemaÁr, looking to the north. CimoÂn LatemaÁr (centre right) and SchenoÂn (right background). Excellent outcrop conditions allow lateral tracing of individual horizons in the lagoonal strata. Ó 1999 International Association of Sedimentologists, Sedimentology, 46, 893±912

Facies architecture of the LatemaÁr

895

Fig. 3. Vertical sections measured in the LatemaÁr Massif and the position of the four marker horizons (I, M, N and R) within the succession. For location names, refer to Fig. 5. Supratidal to subaerial facies and tepees are marked in black; subtidal to intertidal facies in white. Compare stratigraphic subdivision with Fig. 18.

Longobardian

Ladinian

Schlern

Illyrian

Anisian

Hiatus

Fassanian

Platform

Formation

Basin Wengen Group Fernazza /Zoppé / Aquatona Formation

Buchenstein (Livinallongo) Formation

Buchenstein Group

interplatform areas that accumulated hemipelagic, commonly anoxic sediments (Moena Formation; Masetti & Neri, 1980). These ramp deposits form the base of the LatemaÁr build-up. East±west-trending faults truncated the ramp in the late Anisian (Gaetani et al., 1981), forming structural highs upon which the carbonate platforms of the Schlern Formation evolved. The platforms ®rst aggraded as eustatic sea level rose from Illyrian (late Anisian) to early Fassanian times (RuÈffer & ZuÈhlke, 1995), thereby increasing accommodation. In the late Fassanian (late early Ladinian), accommodation was limited on the platform top (Brack & Rieber, 1993), and the platforms prograded into the neighbouring basins (Bosellini, 1984). Basinal facies are represented by the Buchenstein Group, 100±150 m thick, that correspond in age to the platform sediments, which are up to 1000 m thick (Fig. 4; Bosellini & Rossi, 1974). Owing to these grossly different sediment thicknesses, a relief of up to 1000 m existed between platforms and basins at the end of the Buchenstein time (Bosellini, 1984, 1991). In the early Longobardian (early late

Hiatus

Contrin Formation

Moena Formation

Fig. 4. Stratigraphic relationship of the Schlern Formation to adjacent units in the central western Dolomites (after Bosellini, 1991).

Ladinian), carbonate production ceased in the central Dolomites owing to a magmato-tectonic event (Viel, 1979; Bosellini, 1984). This magmatism, lasting until the early Carnian, is characterized by tuf®tes, basalts and pillow lavas of the Wengen Group. Volcanic centres are situated in the Predazzo and Monzoni area (Fig. 1), marked

Ó 1999 International Association of Sedimentologists, Sedimentology, 46, 893±912

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by the radial arrangement of faults and volcanic dykes (Doglioni, 1983) formed by updoming above magmatic chambers. Basinal areas close to these centres are locally ®lled with up to 400 m of volcaniclastics. They onlap slope sediments and cover several carbonate platforms in the Dolomites (Bosellini & Rossi, 1974). Á R Ð A TOPIC OF INTENSE THE LATEMA DEBATE The LatemaÁr Massif is part of a set of atoll-like carbonate platforms and has a diameter of about 2á5 km. The platform succession at the LatemaÁr is 720 m thick and is part of the Schlern Formation (Fig. 4). The platform interior is characterized by a large lagoonal area, which consists of repeated shallowing-upward, decimetre- to metre-scale cycles that, according to Goldhammer et al. (1987, 1990) and Hinnov & Goldhammer (1991), were caused by high-frequency orbitally induced Milankovitch cyclicity. Approximately 600 cycles were reported by these authors, leading to a time span of roughly 12 Myr for the deposition of the cyclic platform interval in the latest Anisian to late early Ladinian. The combination of biostratigraphy and radiometric dating of zircons, however, calls into question this Milankovitch interpretation (Brack & Rieber, 1993; Brack et al., 1996; Mundil et al., 1996). Age-diagnostic fossils, mainly ammonites, and U±Pb ages of zircons in volcaniclastics, intercalated in platform and basin strata, show that the duration of the upper part of the LatemaÁr succession is between 2 and 4á7 Myr. If this is correct, each cycle would have lasted about 5000±7000 years, which is clearly not within the Milankovitch band. Using these dates, we have calculated that compacted carbonate accumulation rates ranged between 100 and 235 mm kyr)1 for the uppermost 470 m of the LatemaÁr platform, which covers the 598 `cycles' reported by Goldhammer et al. (1990). This contrasts with a compacted carbonate accumulation rate of 39 mm kyr)1 using the 12 Myr duration of Goldhammer et al. (1987, 1990). De Zanche et al.'s (1995) biostratigraphic data for the lower, Anisian part of the LatemaÁr platform result in a stratigraphically higher position for the Anisian/Ladinian boundary compared with that determined by Brack & Rieber (1993), such that the Anisian portion reaches a thickness of about 400 m, whereas in several other platforms, the same time interval

is represented by less than 100 m of carbonate deposits (De Zanche et al., 1995). The 400m-thick Anisian succession is encompassed by the avisianum ammonoid subzone, which is considered to have a time span of about 500 kyr (de Zanche et al., 1995). This results in a carbonate accumulation rate of about 800 mm kyr)1, which so surpasses all other estimates that it appears unrealistic. DEPOSITIONAL ZONES The slope and reef environments of the LatemaÁr have been described by Harris (1993, 1994). We focused mainly on the shallow-water environments of the platform top (Fig. 5). The backreef strata can be correlated around the build-up, whereas the sediments of the innermost lagoon have been eroded away.

Reef According to Harris (1993), a shallow subtidal, high-energy, wave-resistant reef only a few tens of metres wide forms the margin of the LatemaÁr platform, and protected the platform interior, although it shows no signs of emergence and is considered to have had low topographic relief. Both the reef at LatemaÁr, as well as Ladinian reef boulders from the nearby Schlern area, which were transported from the upper foreslope, consist of mainly microbial structures and Tubiphytes stabilized by syndepositional marine calcite cements (Brandner et al., 1991; Harris, 1993; Senowbari-Daryan et al., 1993).

Slope Foreslope sediments consist of metre-scale beds with well-cemented blocks of reef- and upper foreslope-derived boundstones with intercalated lagoon-derived litho- and bioclastic debris (Harris, 1994). Upper foreslope clinoforms show inclinations of 25±35°, decreasing downslope. Peloidal bioclastic turbiditic grainstones show distinct ®ning-upward successions and are prominent at the toe of the slope. They become ®ner grained towards the basin and inter®nger with the pelagic sediments of the Buchenstein Formation (Italian: Livinallongo Formation). In contrast to Harris (1994), Blendinger (1994) argued that the foreslope deposits of the time-equivalent Marmolada area nearby were produced almost entirely in situ, such that the slope is, in fact, the



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Fig. 5. General facies distribution of the LatemaÁr platform (explanation in the text).

Lagoonal Facies (Σ=720 m)

platform's most important carbonate factory (Blendinger, 1996; Harris, 1996).

Backreef and lagoon The deposits of the LatemaÁr lagoon were subdivided into a lower Anisian (`Lower Edi®ce') and an overlying, mostly Ladinian part (`LatemaÁr limestone'), the latter consisting of sub- and peritidal cycles (Gaetani et al., 1981). According to Goldhammer et al. (1987, 1990), the deposits are always arranged in the same vertical order, with shallow subtidal facies directly overlain by a subaerial dolomitic or caliche crust. The LatemaÁr limestone facies are described in detail below. MICROFACIES DESCRIPTION OF THE Á R LAGOON LATEMA Based on a study of more than 700 rock samples and 250 thin sections, ®ve microfacies types in the lagoonal suite were recognized, based on primary rather than diagenetic features.

Reefbelt

presumed

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major fault

Microfacies type 1 Dasycladalean algae-bearing peloidal pack- to wackestone generally forms up to 2-m-thick, massive beds, commonly with an erosive base. Microfacies type 1 is volumetrically the most important rock type (Fig. 6). In addition to peloids and subordinate dasycladalean algae, some aggregate grains, lumps, bioclasts and ¯at pebbles up to 15 cm long are locally present. The calcareous algal bioclasts have micritic envelopes. Bioturbation and laminated fenestral fabrics of irregular size (type LF-B of Tebbutt et al., 1965) occur locally.

Microfacies type 2 Peloid-bearing dasycladalean algal packstone differs from microfacies type 1 in having a predominance of dasycladalean algae (Fig. 7), which are commonly aligned parallel to bedding, show micritization at the margins and are locally dissolved, resulting in mouldic porosity. Besides peloids, aggregate grains and lumps are slightly more common than bioclasts and foraminifera.


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Fig. 6. Microfacies type 1 ± Micrite-rich peloid-pack-to wackestone with dasycladalean algae. Scale bar is 1 cm.

Fig. 8. Microfacies type 3 ± pack- to wackestone with peloids and lumps. Scale bar is 1 cm.

Former interparticle pores and fenestrae are ®lled with radial ®brous as well as blocky cements. Beds of microfacies type 2 commonly have an erosional base and are intercalated with, or grade laterally into, microfacies type 1.

amounts of aggregate grains and bioclasts are also present. Laminated fenestral fabrics of the LF-A type (elongated cavities, which are distinctly larger than interparticle porosity, often within micritic carbonates; Tebbutt et al., 1965) are much less common than the LF-B type.

Microfacies type 3 Fenestral pack- to wackestone with peloids and lumps is extensively bioturbated (Fig. 8). Minor

Microfacies type 4 Pack- to grainstone with oncoids, pisoids and aggregate grains, and has allochems up to 5 cm in diameter (Fig. 9); gastropods, other bioclasts and intraclasts are rare. This microfacies type commonly shows an erosive base and mainly inversely graded bedding. Beds dominated by pisoids may comprise caliche crusts (Fig. 10). Primary interparticle porosity is high. Cavities are largely ®lled with inversely graded internal sediments and early diagenetic cements, such as dripstone cements.

Microfacies type 5

Fig. 7. Microfacies type 2 ± dasycladalean algae± peloid packstone. Scale bar is 1 cm.

Diagenetically overprinted, bioclastic grain- to packstone with coated grains occurs in tepee cavities, in metre-sized intertepee depressions and in decimetre-thick red horizons that are traceable across the platform interior. Pisoids, aggregate grains, lumps, ooids and gastropods are the dominant components (Fig. 11), and grainstones are more abundant than packstones. Flat pebbles, ammonites and quartz grains are rare.



Fig. 9. Microfacies type 4 ± pack- to grainstone with oncoids and aggregate grains. Scale bar is 1 cm.

Locally, however, the entire rock is formed of largely unfragmented gastropod shells. Components are commonly reddish in colour and show corrosion features. Mainly interparticle porosity makes up to one-third of the rock volume, with much of the cavities ®lled with reddish marls. Remaining open space was sealed by early diagenetic, often gravitational cements (e.g. dripstones, botryoidal cement, `mamillary crusts'). Sheet cracks are common within and laterally adjacent to tepees, and centimetre-thick crusts of

Fig. 10. Caliche with inversely graded internal sediment. Scale bar is 1 cm.

899

Fig. 11. Microfacies type 5 ± diagenetically overprinted, bioclast-rich grain- to packstone with coated grains. Pores are ®lled with red internal sediments. Scale bar is 1 cm.

radial ®brous cements, as well as red internal sediment, are present inside these cracks. DIAGENETIC OVERPRINTING Early diagenesis resulted in the formation of dolomitic cycle caps, which re¯ect the dolomitization of bank tops (Goldhammer et al., 1987, 1990), tepee structures (Hardie et al., 1986), red exposure horizons and caliche crusts. The composition of the dolomitic caps re¯ects the composition of the underlying carbonate bed in that the components and the amount of carbonate mud are variable, depending on which of the ®ve microfacies types are overprinted. The occurrence of dolomitic cycle tops without associated evaporites argues for an environment that was regularly ¯ooded and not completely subaerial. Therefore, it seems more reasonable to interpret the dolomitic caps as inter- to supratidal rather than purely subaerial (wholly above marine in¯uence), as suggested by Goldhammer et al. (1987, 1990). They emphasized that only subtidal deposits underwent subaerial dolomitization to form dolomitic caps. The dolomitization was linked to rapid eustatic drops rather than to exposure through tidal ¯at aggradation. Given that LF-A porosity is considered one of the few unequivocal criteria to identify intertidal environments (Shinn, 1983), and that


900


these fabrics occur in the upper part of the shallowing-upward cycles, the existence of intertidal facies is likely, perhaps over wide areas of the LatemaÁr. Presumably, a small tidal range (not more than a few decimetres) reduced the width of the intertidal facies belt to a narrow zone. Early diagenetic overprinting, in the form of calichi®cation, vadose cementation, tepee formation and karsti®cation, mask the primary peritidal facies types in many parts of the edi®ce. Caliche crusts in the shallowing-upward cycles at LatemaÁr sometimes coincide with dolomitic caps and are found only in grainstone facies lateral to tepees. The caliche forms irregular centimetre-thick layers, consisting of a basal whitish calcite layer with inverse-graded beds of coated grains, and peloids on top. Roundish vugs up to a few millimetres in diameter probably result from the dissolution of larger components. Some of the grains show coating by whitish calcite similar to the basal band (Fig. 10). The non-dolomitic caliche crusts are interpreted as having formed after dolomitization during prolonged subaerial exposure. The tepees at LatemaÁr are up to 4 m high and several metres in width and comprise up to several limestone beds (Fig. 12). The central cavity between the two inclined ¯anks is ®lled with sediment and/or stalactitic cement and radial ®brous, radiaxial ®brous and botryoidal crusts. Alternating with red marl, the cements generally ®ll more than 50% of the space within the central tepee cavity. Bivalve shells, locally forming coquinas, ammonites and other components characteristic of microfacies type 5, accumulated in intertepee depressions and in tepee cavities. Corrosion features were found on the upper, exposed surface of these ammonites. Tepees have been interpreted as marginal parts of early diagenetic megapolygons, bending upwards as a result of cementation processes (Assereto & Kendall, 1977; Kendall & Warren, 1987). They are thought to grow in peritidal settings (Esteban & Pray, 1983) under seasonally changing environmental in¯uences (Warren, 1983). The red internal sediment within tepees has been interpreted by Gaetani et al. (1981) as soil (terra rossa). Hardie et al. (1986) regarded the red colour as resulting from altered oxidized ma®c mineral grains (probably windblown volcanogenic material). Mutti (1994) interpreted terra rossa of the Ladinian Calcare Rosso (Brescian Alps, Italy) as solution residues that accumulated on platform tops and in dissolution cracks and cavities during

Fig. 12. Tepee structure typical of the LatemaÁr buildup. Example from the lower tepee facies (cf. Fig. 3).

exposure of the platform. The red deposits found at LatemaÁr probably formed as a residue from calcite solution, enriched by wind-transported allochthonous material (e.g. quartz grains) on the exposed platform top. These processes led to the reddening of components, in®ll of open cavities and accumulation and cementation on the sediment surface (microfacies type 5), indicating a supratidal to subaerial setting. Exposure horizons are characterized by a marly matrix and reddish corroded intraclasts, oncoids, pisoids, lumps and peloids. These corrosion features suggest prolonged meteoric in¯uence. A similar sediment, generally with submillimetresized components, commonly ®lls cavities. In contrast to the tepee environment, early diagenetic cements are largely absent.

SHALLOWING-UPWARD CYCLES The early Ladinian succession of the inner part of the LatemaÁr platform is characterized by well-


Facies architecture of the LatemaÁr de®ned shallowing-upward cycles (Fig. 13). The different microfacies types tend to be stacked in the same vertical order. There are, of course, lateral differences in the completeness of the cycles, depending on the depositional location within the platform and the stratigraphic position. We consider cycles with tepees or dolomitic caps at their tops to be the most complete, as these indicate supratidal to subaerial exposure. The dasycladalean algae-bearing peloid packto wackestone (microfacies type 1), forms the base of the idealized shallowing-upward cycle. The relatively high carbonate mud content suggests a low-energy depositional environment. Common erosive bases and occasional large ¯at pebbles argue for at least locally high-energy event deposition, resulting from storm reworking during the transgression. The dasycladalean algae could either have grown in situ or have been transported from the inner lagoon to the place of deposition (maximum of a few hundred metres). In situ growth of dasycladalean algae argues for shallow subtidal conditions (e.g. Johnson, 1961; Ott, 1972). Even if the components are allochthonous, which is indicated to some extent by the varying amount of thalli and by lumps and aggregate grains that occur only locally in this facies, the general absence of subaerial exposure features, such as microkarst or gravitational cements, suggests a subtidal depositional realm. The dasycladalean algae-dominated microfacies type 2 commonly shows aligned dasycladalean algae as well as erosive bases, which argue for high-energy transport processes. As in microfacies type 1, the thalli could have grown in situ or be allochthonous. Alternatively, storms may have removed the carbonate mud of a primary microfacies type 1, resulting in a microfacies type 2 as a `lag deposit'. The close relationship between microfacies types 1 and 2 and, again, the lack of any sign of inter- or supratidal in¯uence argue strongly for a subtidal depositional environment for microfacies type 2. Within the ideal shallowing-upward cycle, microfacies types 1 and 2 are followed by fenestral pack- to wackestone with peloids and lumps (microfacies type 3). Although packstones dominate, suggesting a higher energy environment, wackestones with a high micrite content ± often as high as in microfacies type 1 ± could be taken as an even stronger argument for lowenergy deposition. LF-A porosity, perhaps originating from decaying organic matter or trapped air bubbles in the sediments (Shinn, 1983), requires a tranquil setting. Despite the LF-A

shallow subtidal

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deep

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Oncoids Microbes Dolomite

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Carbonateclassification DUNHAM (1962) & SWANSON (1981)

Limestone, partly recrystallized

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Sedimentary structures Inverse grading Tepee structure

Facies belts Subaerially exposed

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Shallow subtidal, rarely intertidal

Shallow subtidal

Fig. 13. Idealized shallowing-upward cycle of the LatemaÁr lagoon. Legend below for Figs 13, 14 and 15.

porosity, no other features characteristic of interto supratidal settings have been observed. According to this interpretation, microfacies type 3 represents a transitional zone between a shallow subtidal and intertidal environment. The upper part of the idealized cycle consists of microfacies type 4. These pack- to grainstones are interpreted as sediments deposited during moderate- to high-energy conditions. This is indicated


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by the low micrite content and abundant features of high-energy deposition, such as erosional bases and inverse grading of the millimetre- to centimetre-sized components (oncoids, pisoids). The inverse grading can be attributed to sorting caused by wave action (Esteban & Pray, 1983). During high-energy events (storms, strong spring tides), large particles can be moved in the shallow subtidal zone, and waves reach up to the supratidal zone transporting coated grains onto the `beach'. Inverse grading may also be formed in the intertidal realm during fair weather by local sorting processes (Esteban & Pray, 1983). However, calichi®cation may have led to the enlargement of the coated grains. The dripstone cements found in this microfacies type argue for inter- to supratidal, partly vadose (meteoric or marine) conditions after deposition. Additionally, they require a period of quiescence after deposition with enough water to form the gravitational fabric. Tidal currents as well as surf spray may be considered the source of these ¯uids. The top of the idealized cycle is formed by a centimetre-thick dolomitic or caliche crust (dolomitic cap, cf. Goldhammer et al., 1990) and/or a tepee. Microfacies type 5 (diagenetically overprinted, bioclast-rich grain- to packstone with coated grains) only occurs in connection with tepees or decimetre-thick red horizons, traceable across the whole platform interior. It is mainly represented by a diagenetically overprinted and reworked microfacies type 4. The components characteristic of microfacies type 5 indicate highenergy deposition. These processes are also responsible for the occurrence of black pebbles that are interpreted as reworked, subaerially exposed carbonates rich in organic material (e.g. BechstaÈdt & DoÈhler-Hirner, 1983). The depositional setting of microfacies type 5 is thus interpreted as a supratidal or subaerially exposed setting. This is supported by the corrosion of particles, gravitational cements and the reddish argillaceous sediment, resulting from early diagenetic carbonate dissolution. Moderately rounded quartz grains occur in microfacies type 5, especially within the residual material. We interpret these grains as aeolian, as there is no other source in an isolated carbonate platform surrounded by deep basins and with no clastic intercalations. Quartz grains were deposited throughout the development of the LatemaÁr, but only formed a signi®cant percentage of the sediment during periods of reduced carbonate production, such as subaerial exposure, or dissolution of carbonate.

The gastropod coquinas and ammonites trapped in the tepee area support the interpretation of microfacies type 5 as a storm deposit. During high-energy events, bioclasts and other components were transported to the supratidal zone and subsequently altered by diagenetic processes. LATERAL FACIES VARIABILITY WITHIN INDIVIDUAL SHALLOWING-UPWARD CYCLES Shallowing-upward cycles, bounded by marine ¯ooding surfaces at the base and on top, can be traced over several kilometres in the interior of the LatemaÁr build-up, despite lateral facies variations. Figure 14 shows three, 7- to 10-m-thick sections spaced between 1 and 2 km from each other with the exposure horizon M (cf. Figure 3) half-way up. The Valsorda section (Fig. 5) is situated about 50 m from the south-western reef belt. The Forcellone section, 250 m from the reef, is located only a little further away from the reef than the LatemaÁr section (200 m distance). A number of other correlated, traced and measured sections are omitted from Fig. 14 for clarity. Each cycle was traced using marine ¯ooding surfaces as tie-points. It was not possible to trace this interval into the innermost lagoon, as this part of the stratigraphic column has been eroded. The exposure horizon in the central part of the three sections is characterized by laterally uniform subaerial deposits of microfacies type 5. This marker bed is especially well suited for correlation purposes, in contrast to the other intervals, which may show distinct lateral facies changes over a few tens of metres. Four different types of lateral lithofacies changes in the same three sections are shown in more detail in Fig. 15. Starting with the Forcellone section, shallow subtidal sediments of microfacies types 1 and 2 change to the transitional microfacies type 3 (example 1, Valsorda section) and inter- to supratidal microfacies type 4 (example 2, LatemaÁr section). The transitional microfacies type 3 within the Forcellone section grades laterally into microfacies type 4 of shallower, inter- to supratidal environment in the direction of CimoÂn LatemaÁr as well as towards Cima Valsorda (examples 3a and 3b). Beds with early diagenetic dolomitic caps show lateral facies changes into horizons with features of subaerial exposure (example 4a) and tepees (example 4b).


Facies architecture of the LatemaÁr Forcellone section

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3 4 3 3-4 2

1

1-2

3

3

4

2 1-2 4

3

4 1 4

1

5 4 5 3 5

4

4

Fig. 15

4

5 1 5 3 4 4 3

5 1 5

3

Fig. 15

10

5 3-4

3

3-4

903

1

4 4

3 1

1

0m

0m

Fig. 14. Lateral facies changes within traced and correlated beds. This pro®le represents a curved cross-section, starting at the south-western marginal area (Valsorda section, left), cutting an outer part of the lagoon (Forcellone section, centre) and ending at the north-eastern margin (LatemaÁr section, right) of the LatemaÁr build-up. Numbers in the columns indicate microfacies types; triangles indicate shallowing-upward cycles bounded by marine ¯ooding surfaces. Shallow subtidal facies are marked in green; shallow sub- to rarely intertidal facies are striped in green and light blue; inter- to supratidal facies are shown in light blue; subaerially exposed facies are red, and dolomitic crusts are yellow. The sections shown in Fig. 15 are between the horizontal arrows. Legend as for Fig. 13.

Subtidal microfacies types 1 and 2 and the subto intertidal microfacies type 3 dominate in the Forcellone section. In comparison with the LatemaÁr and the Valsorda sections, dolomitic crusts and supratidal sediments occur less frequently. The LatemaÁr and Valsorda sections show more abundant supra- to intertidal facies as well as dolomitic crusts and tepees (microfacies types 4 and 5), whereas the abundance of subtidal facies (microfacies types 1 and 2) is comparatively low. This pattern can be documented in all traced intervals (cf. Fig. 3). FACIES BELTS AND PALAEORELIEF Beds containing supratidal facies with tepees are more common throughout the stratigraphic succession towards the marginal parts of the platform, with a maximum content of reddish horizons and tepees about 50±150 m away from

the reef belt (cf. Fig. 14). The Forcellone section, containing mainly shallow subtidal sediments, represents the overall deepest environment of the three. In contrast, the Valsorda and LatemaÁr sections exhibit a distinctly higher proportion of supratidal facies with tepees. Although it is situated further away from the reef, the LatemaÁr section in the northern part of the build-up contains signi®cantly more supratidal deposits than the Valsorda section from the south-western part. Several other sections also show this asymmetry in facies distribution, suggesting that, in the north and north-east (e.g. CimoÂn LatemaÁr), the supratidal tepee belt was somewhat broader than the equivalent belt in the west (e.g. Valsorda) and south (Cima Feudo, erroneously called Mt Cavignon by Harris, 1993). At Cima di Valsorda, a 250-m-broad tepee belt can be observed, whereas at CimoÂn LatemaÁr, the tepee zone is about 500 m wide. The occurrence of more tepee zones in the north and north-east in


904


Forcellone section

Valsorda section

Latemàr section

example 3a

4

example 3b 5

5

P-G

W-P

3

4

5

P 4 3

4

4

3

4

3

3

3 3 4

P

2

1

2

example 1

3

1-2

W-P

3

P-G

1

5

P-G

4

3

2 1-2

4

P-G 1

0m

4

2

3

4

4 1

3

1-2

3

2

example 2

4

3-4 2

4

example 4a

1

54

1

P 4

example 4b

0m

Fig. 15. Detail of sections in Fig. 14 showing the four main types of lateral lithofacies changes in the LatemaÁr lagoon (examples 1±4). Numbers indicate microfacies types. Legend as for Fig. 13.

comparison with correlative intervals in the south also argues for a topographically more elevated supratidal belt in the north. The transition from lagoonal to reef facies is commonly faulted, but is well exposed at Cima Feudo in the south of the LatemaÁr Group (Fig. 5). A representative palaeotopographic model for the LatemaÁr can be established here (Fig. 16). The palaeobathymetry decreases from the subtidal lagoonal interior towards the supratidal tepee belt. Passing from the heavily cemented tepee belt (`mamillary crusts', botryoidal and radial-®brous cements) towards the reef, the amount of supratidal facies decreases and ultimately disappears slope foreslope

transition zone

belt

suprasubtidal tidal

open marine subtidal inter-

SW

platform interior

windward margin reef belt

tens of metres away from the reef itself, where a narrow subtidal zone occurs. This transition zone (Fig. 16) is characterized by packstones and, in the northern part at CimoÂn LatemaÁr, also by grainstones similar to those found in the lagoon. However, the sediments of the transition zone contain notably more submarine early diagenetic cements (`evinosponges' and `Groû-Oolithe' of some earlier authors) than the deposits of the inner lagoon. The predominantly microbial reef belt is characterized by unstrati®ed boundstones and large amounts of early marine cements. It is interpreted as subtidal, as it lacks any evidence of subaerial exposure (Harris, 1993).

backmargin

lagoon

intertidal

shallow subtidal

leeward margin backmargin

tepee belt

transition zone

inter-

i n t e r t i d a l supratidal subtidal

slope

reef belt

foreslope

subtidal

open marine

NE

wind direction

mean high water mean low water

possible current

marine basin

W/B

P-G wacke- & packstones with dasycladaleans & peloids

B G 0

dolocrust

200

grain- & packstones

P-G

with aggregate grains & oncoids dolomitic crust

tepees

wacke- & boundstones

marine basin

packboundstones with grainstones with grainaggregateabundant cements, internal grains & stones sediment peloids microbes & & Tubiphytes breccia

400m

Fig. 16. Palaeorelief model for the LatemaÁr with schematic facies distribution (vertically exaggerated). Palaeowind direction (south-west to north-east), shown by large arrow, results in asymmetric geometry of the build-up. The tepee belt, the transition zone and the reef are considered to represent the platform margin. Ó 1999 International Association of Sedimentologists, Sedimentology, 46, 893±912

Facies architecture of the LatemaÁr Cementation features in the LatemaÁr lagoon increase towards the margins, being most abundant in the tepee belt, the transition zone and the adjacent reef. The high-energy shoreface deposits of microfacies type 4 in the transition zone indicate that the tepees were in¯uenced by open marine ¯uids from the seaward side. As no variation in cement types can be observed at the seaward and lagoonal sides of the tepee belt, it is likely that normal seawater passing through from either side was responsible for the cementation processes within tepees. In terms of palaeorelief, the supratidal tepee belt alone has to be considered the platform margin, which protected the platform interior from wave energy. However, as the reef, the transitional zone and the tepee belt are characterized by heavy cementation, the three together stabilized the outer part of the build-up and can be considered the platform margin (Fig. 16). FORMATION OF TEPEES AND EXPOSURE HORIZONS The characteristic cements of the tepee cavities require an episodic or continuous ¯ow of calciteoversaturated waters through the sediments. Recent tepee settings are located topographically below sea level, resulting in a constant ¯uid ¯ow from open marine areas towards the lagoon (Handford et al., 1984). In the LatemaÁr lagoon, in contrast, there are no indications of a difference in sea level between the lagoon and the open sea. On the contrary, the accumulation of ammonites in Anisian strata in the eastern part of the LatemaÁr lagoon (Brack & Rieber, 1993; Brack et al., 1996) suggests an inlet that permitted free water exchange with the surrounding basin. The ammonites were probably transported into the lagoon by in¯owing currents. They can be observed over several metres of thickness and tens of metres laterally interbedded in muddy carbonate deposits. It is assumed that inlets and the interpreted horseshoe shape of the build-up persisted during most of the LatemaÁr's development.

Lateral facies changes from tepees to bedded facies Lagoonwards, most tepees, which often incorporate earlier formed dolomitic crusts into their ¯anks, grade into peritidal bedded facies with or without dolomitic caps. The intensive cementation of the tepees and the observed lateral

905

changes into facies without signs of subaerial exposure argue against the emergence of lagoon sediments during tepee formation. As the tepees are characterized by red residual sediments, a period of subaerial exposure is thus proposed for the tepee belt. The observed synsedimentary carbonate solution features in microfacies type 5 and the formation of red residual sediment are interpreted as an effect of meteoric waters. Marine waters probably percolated into the tepee belt from both open marine and lagoonal directions (Fig. 17A). Fluid ¯ow towards the tepee belt depends on the position of sea level: when the tepee belt became partly exposed during a regression, cementation and dissolution processes started, leading to the formation of tepees. When the tepee belt was ¯ooded again during transgression, subtidal conditions were reestablished, and tepee formation stopped. Repeated exposure led to the formation of tepee stacks with decreasing ¯ank angles upsection (Fig. 12). According to our model, the areal extent of an individual tepee belt re¯ects the duration of the mutual in¯uence between carbonate solution and precipitation during that episode. Therefore, tepees mirror not only the palaeotopography of the build-up, but also provide clues to sea-level ¯uctuations. The tepee belt was most extensive during periods when the sea level was low, but marine conditions still existed in the lagoon.

Lateral facies changes from tepees to exposure horizons While most LatemaÁr tepees grade laterally into peritidal bedded facies, a small number of marginal tepee zones of only a few decimetres in thickness pass laterally into traceable platformwide exposure horizons. Owing to the absence of cement layers within subaerially exposed facies, it is assumed that insuf®cient marine water was available for precipitation of large amounts of calcite at and near the surface (Fig. 17B). The decreased abundance of tepee structures at this time suggests that the lagoon dried out rapidly and completely during the sea-level lowstands, producing lagoon-wide meteoric karsti®cation. This is represented by the corrosion features and red residual internal sediment. Carbonatesupersaturated waters could reach the tepee belt only from the open sea and not from the lagoon, forming individual tepees. Exposure horizons were therefore present in the lagoonal area during intervals when tepees were restricted to a very narrow tepee belt.


906


A

evaporation

meteoric waters tepees

mean sea-level

surf spray

percolating marine waters


open marine basin

tepee belt = topographic platform margin

lagoon 0

50

100m

B

meteoric waters

evaporation mean sea-level

red exposure horizon

surf spray


open marine basin

small individual tepees

lagoon

tepee belt = topographic platform margin 0

50

100m

Fig. 17. (A) Tepee formation during high-frequency sea-level lowstand with in¯uence of marine waters from both open marine areas and the lagoonal interior. Synsedimentary carbonate solution features and residual sediment resulted from the in¯uence of meteoric waters. (B) Subordinate tepee formation during high-frequency sea-level lowstand with complete drying of the lagoonal interior. Marine waters percolate only from open marine areas into the tepee belt. Synsedimentary carbonate solution by meteoric waters results in extended meteoric karsti®cation, which is indicated by a centimetre- to decimetre-thick exposure horizon.

ASYMMETRY IN FACIES ARCHITECTURE

the north-eastern slope shows few of these features.

Along with the varying width of the tepee belt, distinct differences in the width of the reef belt are observed, further supporting the asymmetric character of the facies bodies on the build-up. At Cima Feudo in the south and at the Reiterjochspitze (Fig. 5) in the western part of the LatemaÁr Massif, the reef belt is only about 30 m wide, whereas in the north and north-east (at SchenoÂn), it spans about 50±70 m. However, the reef facies remains rather uniform in composition. Furthermore, the slope angle is generally lower in front of the wide tepee belt in the north and north-east. Megabreccias resulting from slope failure are present mainly in the west of the build-up at the Reiterjochspitze±Eggentaler Horn, whereas

The asymmetry of the tepee belt caused by windward/leeward effects Asymmetry in facies distribution may be explained by palaeoceanographic and/or palaeoclimatic factors, such as current and wind directions. The northern and north-eastern parts of the LatemaÁr platform are interpreted as leeward, and the southern and western part as windward (Fig. 16), in accordance with the envisaged prevailing palaeowind direction of the Dolomites (Blendinger & Blendinger, 1989). This is supported further by a computer-based reconstruction of palaeowind directions for this marginal area of the Tethys. The modelling of palaeoclimatic data


Facies architecture of the LatemaÁr from the western part of the Tethys yields prevailing wind directions coming seasonally from the south-south-west and north, which correspond to the south-east and west for the present orientation of the Dolomites according to the palaeogeographic model of Scotese & McKerrow (1990). Storms also had an impact on the platform. Cyclones may have formed in the closed end of the Tethys to the west and in¯uenced the deposition and transport of sediments on the carbonate platforms in the Dolomites (cf. Blendinger & Blendinger, 1989). Squalls and thunderstorms, which coincide with high spring tides, are also able to move a signi®cant amount of sediment (cf. Shinn, 1968). Sediment-laden waters could have been driven preferentially towards the lee side of the build-up, which led to the formation of a wider and topographically higher tepee belt in the north and north-east of the LatemaÁr build-up, where sediment was trapped and cemented in tepees. A signi®cantly higher proportion of transported lagoonal debris in this area compared with the tepee belt in other marginal areas of the LatemaÁr supports our windward/leeward model. The slope area only received insigni®cant amounts of lagoonal debris (Harris, 1994; cf. Blendinger, 1994), as the elevated tepee belt functioned as a barrier, hindering off-platform transport.

Potential causes of reef asymmetry Nutrient-rich current from the north-east promoted reef growth The asymmetry of the LatemaÁr reef might, alternatively, have resulted from a nutrient-rich current from the north-east, opposite to the assumed wind direction. This would have boosted the growth of microbial reefs in the north-east, whereas the reef margin in the south-west was sheltered behind the Agnello platform (cf. Fig. 1), which formed a protective barrier against wind-driven currents from the south-west, thus inhibiting signi®cant reef growth at this margin (Blendinger & Blendinger, 1989).

Megabrecciation hampered extensive reef development in the south-west The slope deposits of the LatemaÁr are composed almost entirely of mud-free sediments (Harris, 1994). Sediment fabric, rock strength and height of slope are regarded as the major controls on the

907

slope angle of carbonate platforms (Kenter, 1990; Purdy & Bertram, 1993). Grain-supported fabrics, as in the LatemaÁr, reach a signi®cantly higher slope angle than typical mud-supported fabrics (Kenter, 1990). Rock strength is further enhanced by submarine lithi®cation (Schlager & Camber, 1986), which played an important role at the LatemaÁr as well. Purdy & Bertram (1993) calculated the height for slope failure of carbonate platforms with an initial slope of 20°. They found critical values at about 850±1500 m of slope height. The slope angles of the LatemaÁr build-up vary between 25° and 35° (Kenter, 1990; Harris, 1994), and the basin reached depths of 800± 1000 m (Bosellini, 1984, 1991). Therefore, rock slides and collapse of the foreslopes should have been common around the LatemaÁr platform. Predictably then, the slope at the Reiterjochspitze±Eggentaler Horn in the west shows extensive megabrecciation, which may have hampered the development of a broader margin. In contrast, in the north and north-east, megabreccias are smaller, and the slope inclination is lower.

Platform progradation of the margin led to a broader reef in the north-east Opinions differ as to whether the LatemaÁr platform prograded or not. Of¯ap geometries have been described by Bosellini (1984) from the Reiterjochspitze in the west and from the area of the SchenoÂn in the north of the platform. According to our observations, the western, as well as the southern reef belt at Feudo, do not show progradation. In the north and north-east, the situation is unclear. Fault contacts exist between lagoonal-to-reef and reef-to-slope facies, and no progradation can be observed directly. However, differences between the slopes in the north-east and west may argue for a dominant progradation towards the north-eastern part of the platform (cf. Goldhammer & Harris, 1989). In the latter area, the distance between the inter®ngering of slope and basin facies and the actual platform margin is distinctly greater than in the stratigraphically equivalent part of the build-up in the west. This distribution, and the comparatively low inclination of the platform in front of the wide north-eastern tepee belt, can be explained by a shallower basin on this side of the platform and might have led to the development of a broader reef belt. On the other hand, the apparent asymmetry of the LatemaÁr reef might also result from a comparison of reef segments from different stratigraphic levels. In this case, our observations


908


would indicate an overall broadening of the reef belt towards the end of the Ladinian, assuming a rather uniform time-equivalent reef distribution.

VERTICAL VERSUS LATERAL FACIES CHANGE

Stratigraphic subdivision of the LatemaÁr buildup In comparing different parts of the lagoonal succession of the LatemaÁr, there are notable differences in facies between stratigraphic levels. Parts of the succession, some tens of metres thick, show a higher percentage of inter- to supratidal facies than other parts, which are dominated by subtidal sediments. These differences clearly represent long-term changes in palaeobathymetry that can be observed and correlated between the tepee belt and the lagoon. An exclusively autocyclic model (Pratt & James, 1986) for the interpretation of the LatemaÁr shallowing-upward cycles can be ruled out. Platform-wide subaerial exposure horizons cannot be formed purely autocyclically, and the presence of subtidal facies involved in the tepees indicate relative sea-level fall and an allocyclic origin of the small-scale cycles. Long-term changes in palaeobathymetry are related to allocyclicity as well. Goldhammer et al. (1990) interpreted the vertical long-term facies changes as attributable to third-order relative sea-level changes. The impact of these sea-level ¯uctuations on different parts of the platform was, however, different, owing to a palaeotopographic relief. This has implications for the interpretation of subsurface reservoir architecture, as lateral variabilities in microfacies and porosity, as a result of palaeobathymetry, might not be recognized easily. Our subdivision of the LatemaÁr succession (Fig. 18) differs slightly from that of Goldhammer et al. (1990). The basal part is characterized by an entirely subtidal package, which we term the lower platform facies (LPF; cf. Goldhammer et al., 1987). The overlying package consists of a facies similar to the LPF, but showing discrete tepee horizons about every 10 m. These, up to several metres high, so-called `submarine' tepees (terminology of Goldhammer et al., 1990) are in our opinion also supratidal tepees displaying the same characteristics as the tepee structures further upsection. They comprise several subtidal limestone beds with ¯ank angles decreasing

Fig. 18. Stratigraphic subdivisions of the LatemaÁr succession according to Goldhammer et al. (1990), on left, and according to this study, on right. Anisian/Ladinian boundary, at right, according to Brack & Rieber (1993).

upwards. These tepee stacks (Fig. 12) characterize the LTF (lower tepee facies). This interval is well exposed in the south-west of the platform and at Zan de Montagna (Fig. 5). The typical red residual sediments in tepee cavities are also present in this mostly subtidal stratigraphic unit. The upper part of the LatemaÁr succession comprises four units with peritidal facies, differing in the abundance of tepees and supratidal sediments. In this part of the succession, a lower and an upper cyclic facies (LCF and UCF) can be distinguished from a middle and an upper tepee facies (MTF and UTF). In LCF and UCF times, tepees are restricted to the tepee belt, whereas peritidal facies dominate the lagoonal interior. On the other hand, MTF and UTF intervals show abundant tepee structures in different parts of the lagoon. The occurrence of a thick, prominent tepee unit (UTF) at the top of the succession at CimoÂn LatemaÁr (Egenhoff & PeterhaÈnsel, 1995) was overlooked by Goldhammer et al. (1990), despite its importance in the reconstruction of the third-order relative sea-level curve. The UTF unit shows that the long-term sea-level trend in the uppermost part of the LatemaÁr succession is not transgressive, as assumed by Goldhammer et al.


Facies architecture of the LatemaÁr (1994), but is instead regressive. In addition, the thickness of the stratigraphic succession (670 m) measured by Goldhammer et al. (1990) differs from our results (720 m) by 50 m (Fig. 18), owing to their incorrect correlation across the faults between the tectonic blocks.

Sea-level changes Vertical facies differences indicate changes in palaeobathymetry, which are best explained by sea-level ¯uctuations. The following interpretation is based on measured sections covering the whole stratigraphic succession of the LatemaÁr. No additional analysis of accommodation (cf. Goldhammer et al., 1994) has been made at the current stage of investigation. The still undetermined controls on cyclicity (Milankovitch or not), and the question whether every cycle spanned the same time interval, makes an accommodation analysis more dif®cult. On top of the Contrin ramp, the subtidal deposits of the LPF represent a high-amplitude relative sea-level rise, which lasted approximately until the lower part of the MTF. Subtidal facies decrease upwards within the LTF and grade into the peritidal cycles of the LCF, both indicating a decelerating rate of relative sea-level rise. From the base to the centre of the MTF, the proportion of inter- to supratidal facies with abundant tepees increases, indicating a decrease in water depth, probably because of a relative sealevel stillstand or fall. Subtidal facies become progressively more widespread during the upper MTF and the overlying UCF, which is interpreted as a renewed relative sea-level rise. The UTF at

909

the top of the LatemaÁr succession includes a relatively higher proportion of tepees and supratidal facies, representing another relative sealevel stillstand or fall. The reconstruction of long-term palaeobathymetrical changes suggests an explanation for the formation of the LTF tepees and the exposure horizons. Whereas the LTF tepees are found within a generally subtidal facies, the MTF and the UTF show tepees in a peritidal environment. The tepees of the LTF occur about every 10 m, as do the exposure horizons in the upper, peritidal part of the succession. Sea-level ¯uctuations of fourth order (cf. Goldhammer et al., 1990) are probably responsible for the formation of the exposure horizons and the tepees of the LTF. The following model (Fig. 19) explains the formation of both features (cf. Goldhammer et al., 1990). During the initial third-order relative sea-level rise, no high-frequency lowstand was able to expose the platform top (LPF without emergence). In LTF time, as a result of the subsequent reduction in water depth (falling third-order relative sea level), highfrequency cycles of fourth order were able to expose the outer parts of the lagoon, leading to intense tepee development. Owing to the continuous fall of third-order relative sea level during LCF to UTF, every fourth-order lowstand led to subaerial exposure of the entire lagoon. Tepees of all three tepee facies units are generally related to the same processes and formed in a similar environment (cf. Assereto & Kendall, 1977). Whereas in the LTF, a fourthorder lowstand resulted in tepee formation, the tepees of the MTF and UTF represent lowstands

sea-level

lagoon surface

without subaerial exposure Lower Platform Facies

giant tepees Lower Tepee Facies

exposure horizons Lower Cyclic Facies

falling 3rd order sea-level

exposure horizons Middle Tepee Facies

exposure horizons Upper Cyclic Facies

exposure horizons Upper Tepee Facies

rising 3rd order sea-level falling 3rd order sea-level

Fig. 19. High-frequency cyclicity (schematic, probably fourth order) superimposed on third-order relative sea-level ¯uctuations, leading initially to the formation of giant tepees and with continuous shallowing of the lagoon to platform-wide exposure horizons. Ó 1999 International Association of Sedimentologists, Sedimentology, 46, 893±912

910


of ®fth order that did not coincide with a fourth-order lowstand, which would lead to an exposure horizon. CONCLUSIONS Our modi®ed stratigraphic subdivision for the lagoonal succession of the LatemaÁr build-up is based on laterally traced and correlated sections. It takes into consideration the palaeorelief of the build-up and long-term bathymetrical development throughout the entire succession re¯ecting changes in relative sea level. In contrast to previous work, an additional interval with tepees is identi®ed at the top of the LatemaÁr succession. The whole succession therefore consists of a basal subtidal lower platform facies, an overlying subtidal package with sporadically occurring tepee stacks (lower tepee facies), two peritidal successions with tepees only in marginal areas (lower and upper cyclic facies) and two peritidal intervals with abundant tepees (middle and upper tepee facies). The sediments of the lagoonal part of the LatemaÁr build-up show a pronounced cyclic architecture, as described by earlier workers. The tracing of four marker horizons over large parts of the preserved platform top reveals distinct facies variations for two stratigraphic units, the middle tepee facies and the upper cyclic facies. Facies analysis of a succession of shallowing-upward cycles allows a bathymetrical interpretation of the lateral facies heterogeneity resulting from a depositional palaeorelief. The zone of highest elevation of the LatemaÁr platform is represented by a supratidal tepee belt, situated in the backreef area. The supratidal tepee belt, the subtidal reef and a transitional zone in between played a major role in stabilizing the platform, as a result of intensive early cementation in particular, and the three environments together are therefore considered the platform margin. The asymmetry in the width of facies belts within individual cycles suggests that the facies distribution in the lagoon was in¯uenced by windward/leeward effects. Dominant storm paths probably winnowed and redistributed lagoonal material, resulting in a broad tepee belt in the north-east and a relatively narrow one in the south-west and west. Tepees indicate episodic subaerial exposure during high-frequency sea-level oscillations without total subaerial exposure of the lagoon. A rapid

emergence of the entire lagoon would not have produced a broad tepee belt, but rather an exposure horizon related to extensive karsti®cation. Given the palaeorelief on the build-up top, the width of the tepee belt is interpreted as re¯ecting the maximum extent of exposure during a high-frequency sea-level fall. Small-scale lateral facies and porosity heterogeneity might easily be overlooked during subsurface reservoir analysis. The high resolution of our data provides a ®rst step towards a better understanding of subsurface analogues of isolated carbonate platforms. ACKNOWLEDGEMENTS We particularly thank Shell Research and Technology Services in Rijswijk, The Netherlands, for their support of our work at the LatemaÁr Massif. Ideas have been sharpened by fruitful discussions with V. Vahrenkamp, P. Brack, R. Mundil, H. Rieber and many other colleagues. The suggestions made by the reviewers, T. Sami and M. T. Harris, as well as A. G. Plint and B. R. Pratt, have helped to improve the quality of the manuscript. Finally, we would like to thank the Gabrielli family, particularly Tonio, for friendship and logistical assistance (e.g. funicular transport of rucksacks, continuing grappa supply) at the `Rifugio Torre di Pisa'. REFERENCES Assereto, R. and Kendall, C.G.St.C. (1977) Nature, origin and classi®cation of peritidal tepee structures and related breccias. Sedimentology, 24, 153±210. BechstaÈdt, T. and DoÈhler-Hirner, B. (1983) Lead±zinc deposits of Bleiberg±Kreuth. In: Carbonate Depositional Environments (Ed. by P.A. Scholle, D.G. Bebout and C.H. Moore), Mem. Am. Assoc. petrol. Geol., 33, 55±63. Blendinger, W. (1994) The carbonate factory of Middle Triassic buildups in the Dolomites, Italy: a quantitative analysis. Sedimentology, 41, 1147±1159. Blendinger, W. (1996) The carbonate factory of Middle Triassic buildups in the Dolomites, Italy: a quantitative analysis ± reply. Sedimentology, 43, 402±404. Blendinger, W. and Blendinger, E. (1989) Windward± leeward effects on Triassic carbonate bank margin facies of the Dolomites, northern Italy. Sedim. Geol., 64, 143±166. Bosellini, A. (1984) Progradation geometries of carbonate platforms: examples from the Triassic of the Dolomites, northern Italy. Sedimentology, 31, 1±24. Bosellini, A. (1991) Geology of the Dolomites ± an introduction. Dolomieu Conference on Carbonate


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