New approach for quantifying water depth applied to the enigma of drowning of carbonate platforms Gianni Mallarino* Dipartimento di Geologia, Universita` di Palermo, via Archirafi 22, Palermo 90123, Italy Robert H. Goldstein Department of Geology, University of Kansas, 1475 Jayhawk Boulevard, 120 Lindley Hall, Lawrence, Kansas 66045, USA
Pietro Di Stefano Dipartimento di Geologia, Universita` di Palermo, via Archirafi 22, Palermo 90123, Italy ABSTRACT This research illustrates application of a fluid-inclusion technique for quantifying water depth of ancient carbonate platforms. Jurassic limestones of Monte Kumeta, Italy, were cemented with submarine calcite during a transition to carbonate platform termination. The calcite cements contain fluid inclusions consisting of Jurassic seawater and immiscible gas bubbles trapped during the growth and penecontemporaneous recrystallization of the cements. Crushing analysis indicates that gas bubbles are under pressures indicative of entrapment in water depths of 23–112 m. Assuming simple deepening and acknowledging chronostratigraphic errors, rates of relative rise in sea level were initially less than 7 m/ m.y. followed by a rate of at least 33 m/m.y. These slow rates are evidence that the platform’s demise was caused by an environmental perturbation other than rapid sealevel rise. The facies transitions and regional studies indicate that the perturbation resulted from nutrient excess or eutrophication in shallow water followed by deepening into ephemeral dysoxic waters at depths perhaps as shallow as 23 m.
STRATIGRAPHY Our analyses were performed on carbonate strata deposited during a transition to termination of growth of a carbonate platform, from a locality at Monte Kumeta in western Sicily. Recent studies have provided detailed data on facies architecture and biostratigraphy based on ammonites and brachiopods (Di Stefano et al., 2002). Unconformities, ferromanganese crusts, pelagic rosso ammonitico sedimentation, and a crinoidal transitional facies signal termination of shallow-water benthic production followed by eventual deepening of the water (Fig. 1). The main challenge is to determine why this termination occurred.
Keywords: paleobathymetry, fluid inclusions, carbonate platforms, Tethys, Jurassic, drowning. INTRODUCTION In studies of ancient marine limestones, one of the great unknowns is the water depth of sedimentation and early marine diagenesis. This problem is particularly apparent in studies of the demise of ancient carbonate platforms, where a goal has been to understand whether extremely rapid rates of relative rise in sea level or environmentally driven decrease in carbonate production led to cessation of shallow-water sedimentation. Although for many drowned platforms this question has been answered (Dominguez et al., 1988; Hallock et al., 1988; and many others), controversies remain for others (e.g., Jurassic Tethyan platforms; Winterer and Bosellini, 1981; Bice and Stewart, 1990; Zempolich, 1993; Galluzzo and Santantonio, 2002). This study illustrates a new quantitative approach for reconstructing water depth in ancient carbonate platforms by using fluid inclusions as a geobarometer for submarine cements. Its utility is demonstrated by interpreting the origin of the drowning phase of a Jurassic carbonate platform from Monte Kumeta, Italy. Many carbonate platforms in the Jurassic Tethys sea underwent a cessation in normal benthic sediment production, followed by an unconformity and pelagic sediment deposition. Some workers have considered that this platform drowning resulted from pulses of rapid tectonic subsidence and eustatic sealevel rise that submerged platforms deeply enough to terminate production of benthic car*E-mail:
[email protected].
bonate sediment (Schlager, 1981; Winterer and Bosellini, 1981; Roux et al., 1988). Analyses of modern carbonate systems and other studies of Jurassic platforms suggest that some ecologic perturbation in shallow-water productivity was needed independent of high rates of relative rise in sea level (Hallock and Schlager, 1986; Cobianchi and Picotti, 2001). Thus it is crucial to quantify water depth and the rate of relative sea-level change to understand which factors were important. Quantifying water depth in ancient carbonate rocks has been a difficult task, but it remains one of the most important variables in paleoenvironmental reconstruction and sequence stratigraphy. Normally sedimentologic and paleontologic comparisons to modern environments are the most common tools, but their applicability is fraught with assumptions because direct modern analogs are typically lacking. In other cases, preserved paleotopography has been used (Goldstein and Franseen, 1995), but application is limited to areas in which outcrops are exceptional and deformation is minor. For the Jurassic rocks discussed here, problems with water-depth interpretations have led to significant disagreement (Garrison and Fischer, 1969; Farinacci et al., 1981; Winterer and Bosellini, 1981; Bice and Stewart, 1990; Zempolich, 1993; Winterer, 1998). Here we present a possible alternative to the classic and often controversial approaches used for interpreting water depth, the first application of a fluid-inclusion geobarometer in marine calcite cements.
PARAGENESIS AND CATHODOLUMINESCENCE Petrography The crinoidal limestones were deposited after the normal shallow-water platform deposition had ceased. These limestones were cemented by syntaxial calcite overgrowths predating the infiltration of reddish micrite, the first deposits of rosso ammonitico in the area (Fig. 1). The overgrowths preserve complex growth zonation grouped into three major growth zones (Fig. 2). Zone 1 has an inner subzone with fine dull orange to nonluminescent concentric growth zones and a thick nonluminescent growth zone (Fig. 2, subzones b and c, respectively). Some inner areas have patchy areas of different luminescence that cut across the concentric zoning (Fig. 2, subzone a). These patches have the same luminescence as the external subzone of zone 1 (Fig. 2, subzone d), which has finely banded dull orange luminescence. Zone 2 consists of thinly banded bright yellow to dull yellowish luminescent concentric growth zones and zone 3 is nonluminescent. Interpretation The cathodoluminescence patterns suggest that in zone 1 the inner subzone a recrystallized during precipitation of the outer subzone d (early recrystallization), pointing to preservation of an early marine composition, rather than later stabilization. Stratigraphic relationships indicate precipitation, and some recrystallization, after deposition of Carixian–early Domerian crinoidal limestones and before infiltration of reddish micrite (rosso ammonitico).
q 2002 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or
[email protected]. Geology; September 2002; v. 30; no. 9; p. 783–786; 4 figures.
783
Figure 1. Jurassic stratigraphy of Monte Kumeta. Facies show transition from normal shallow-water platform deposits (Hettangian and Sinemurian) to pelagic rosso ammonitico deposits. Earliest Carixian (early Pliensbachian) grainstonepackstone is overlain by Carixian–early Domerian (early to earliest late Pliensbachian) crinoidal grainstone-packstone, which represent transitional facies. Magnifying glass enlarges crinoid fragments of crinoidal limestones that have been overgrown by syntaxial calcite cements. Cement growth was terminated by infiltration of reddish micrite related to rosso ammonitico. Solution and bioerosion unconformity truncates these sediments. Rosso ammonitico of early Toarcian age (H. serpentinum zone) deposited on unconformity is truncated by another unconformity and subsequently encrusted with ferromanganese minerals. Succession was eventually overlain by rosso ammonitico of Bajocian age. Synchronous neptunian dikes show that strata were deformed during deposition of succession.
FLUID INCLUSIONS The three growth zones contain fluid inclusions with distributions closely related to the cathodoluminescence patterns of the overgrowth calcite. These distributions indicate inclusion entrapment during precipitation of all three growth zones, entrapment being most common in zone 1. In zone 1, inclusions have highly variable liquid-to-gas volume ratios, ranging from all liquid to gas dominant. This variation could be interpreted to indicate (1) entrapment of liquid and separate gas phases at low temperature, or (2) necking down of high-temperature fluid inclusions to form multiple inclusions after cooling (Goldstein and Reynolds, 1994). However, because all-liquid inclusions are not paired with vapor-rich inclusions and because the volume of gas is high, we conclude that the variable ratios of liquid and gas could not have resulted from necking down of two-phase inclusions. Thus, they resulted from entrapment of liquid and gas bubbles during formation of zone 1, indicating that immiscible gas bubbles were 784
clinging to the surface of the crystals as they grew. This conclusion is further supported by observations in zones 2 and 3, which contain only all-liquid inclusions and have similar sizes and shapes to those inclusions in zone 1. All-liquid inclusions point to low-temperature entrapment, below ;50 8C. The absence of small vapor bubbles indicates that these inclusions have survived the moderate heating of the 2 km of burial without thermal reequilibration. This is also strong evidence against necking down for the origin of variable liquidto-vapor ratios in zone 1, which would have required thermal reequilibration to produce densities or compositions capable of producing bubbles during cooling. If inclusions in zones 2 and 3 have not reequilibrated, it stands to reason that inclusions in zone 1 have not reequilibrated. To evaluate the origin of the syntaxial cements and their inclusions, fluid inclusions were frozen and final melting temperatures of ice (Tm,ice) were measured. Tm,ice values were attained on 34 fluid inclusions from all three
Figure 2. Cathodoluminescence photomicrograph of overgrowth cements in crinoidal limestone showing three main zones. Zone 1 consists of early subzone disrupted by patchy areas (a) with luminescence similar to later phase of growth of zone 1. Rest of zone 1 consists of fine dull orange to nonluminescent growth zone (b), nonluminescent growth zone (c), and finely banded dull orange luminescence growth zones (d). Zone 2 has thinly banded bright yellow to dull yellowish luminescent growth zones. Zone 3 is nonluminescent. Scale bar is 100 mm.
zones. Values are narrowly distributed (mean 5 21.9 8C, s 5 0.17 8C). These values indicate cement precipitation from a fluid with salinity of 35‰ (seawater salt equivalent) (Johnson and Goldstein, 1993) and thus that the overgrowth cements precipitated (and partially recrystallized) in seawater after Carixian– early Domerian crinoidal deposition and before infiltration of rosso ammonitico. To measure pressure within the two-phase fluid inclusions of zone 1 (Fig. 3), the inclusions were opened by crushing while immersed in glycerin (Roedder, 1970). The crushing stage was a simple mechanism (e.g., Goldstein and Reynolds, 1994) mounted on a transmitted light microscope. Every inclusion was recorded on videotape throughout each crushing run. Typical behavior upon crushing was the immediate expansion of the bubble. Bubble sizes were measured before and after the crushing run to determine inclusion pressure before the crush. Of ;300 crushing runs, 31 were successful in recording the bubble before and after the crush. Because of the great difficulty inherent in acquiring these data, it was impossible to compare multiple analyses from the same field of view. Bubble sizes and shapes were measured to determine bubble volume. With repeated measurements of the video records of bubbles on the monitor, it was found that error in the measurement was GEOLOGY, September 2002
Figure 3. Transmitted light photomicrograph of fluid inclusions in zone 1. Note that some inclusions contain bubble (arrow) and others are all liquid. Scale bar is 25 mm.
60.5 mm, which contributed a larger percent error to the smaller bubbles than to the large ones. This measurement error is much larger than that associated with approximations made about bubble shape to calculate volume. Pressures were calculated using Boyle’s law and resulted in values ranging from 3.3 to 12.2 atm (Fig. 4). Because the temperature of original entrapment was close to the temperature at which the crushing runs were conducted, these pressures are representative of Jurassic entrapment pressures (e.g., Goldstein and Reynolds, 1994; Newell and Goldstein, 1999). In modern tropical marine environments, seawater temperature normally is 15–30 8C in the uppermost 200 m of the water column (James and Choquette, 1990). Crushing runs were done at the lab temperature of ;25 8C, close to the temperature of inclusion entrapment. Because the pressure at the time of entrapment was hydrostatic and similar to lab temperature, the density of seawater can be used to convert inclusion internal pressure directly to depth of entrapment. Calculated water depths are as shallow as 23 m (21–25 m for point 1; Fig. 4) and as deep as 112 m (94–171 m for point 31; Fig. 4). DISCUSSION Because the crinoidal limestones were deposited after typical shallow-water platform limestones and before pelagic deposits, it is reasonable to assume deposition along a deepening trend. Therefore, the fluid-inclusion data would indicate crinoidal deposition and the transition to platform termination in depths less than 23 m (21–25 m). The biostratigraphic data suggest that crinoidal limestone deposition took place from mid-late Carixian to GEOLOGY, September 2002
Figure 4. Inclusion pressure data and their conversion to water depths. Horizontal axis arranges data in order of increasing pressure because technique does not allow data to be ordered paragenetically. Error bars are based on measurement error and are controlled by bubble size and pressure.
early Domerian time. When the thickness of transition strata (peloidal grainstone-packstone and crinoidal deposits; average of 7 m) is added to 23 m, and the corresponding time interval of ;4.5 m.y. (based on the time scale of Gradstein et al., 1994) is considered, it leads to a maximum rate of relative sea-level rise of 7 m/m.y. Recent suggestions for modification of the Jurassic time scale might indicate that the rate could be even slower (Pa´lfy et al., 2000). The range in values in the cements shows that after crinoidal deposition, depth may have increased from 23 m (21–25 m) to 112 m (94– 171 m) before infiltration of the rosso ammonitico. It is possible that these depth data fit into one or several parts of a complex sealevel history. However, if one assumes simple deepening, then they indicate a relative rise in sea level of at least 89 m (69–150 m) after crinoidal limestone deposition and before rosso ammonitico infiltration. Using the interval from the beginning of middle Domerian to lower Toarcian (beginning of the H. serpentinus zone) gives a maximum time interval of 2.7 m.y. (error bars not included; Gradstein et al., 1994), yielding a rate of relative sea-level rise of at least 33 m/m.y. (25.5–55.5 m/m.y.). For both intervals, calculated rates of relative sea-level rise are slower than the ;100 m/m.y. potential for Holocene shallow-water carbonate deposition, based on the scaling trend for intervals of 106 yr (Schlager, 1999). Thus, these rates do not support interpretations that termination of this carbonate platform was caused by extremely rapid rates of tectonic subsidence that outstripped the deposition rate of a normal, shallow-water carbonate platform. On the contrary, they suggest
that at Monte Kumeta, during Carixian–early Domerian time, some environmental perturbation could have greatly inhibited carbonate production, leading to platform termination, possibly during a time of slow rates of relative sea-level rise. Some authors have suggested eutrophication as an environmental perturbation that might have led to platform termination. Increased trophic resources (nutrients and organic matter), leading to the suggested oxygendeficient or eutrophic conditions, can be related to upwelling, episodic overturn of deeper nutrient-loaded waters, sea-level falls, land-derived sources, and climatic changes (Hallock and Schlager, 1986; Zempolich, 1993; Cobianchi and Picotti, 2001; Galluzzo and Santantonio, 2002). At Monte Kumeta the bloom of crinoid productivity (transition facies) represents faunas dominated by suspension feeders after normal, shallow-marine phototrophic productivity had ceased. Because our data suggest that this transition took place in water depths shallower than 23 m, it cannot be ascribed to deep-water conditions. The transition could, however, reflect nutrient overload leading to eutrophication, degradation of phototrophic productivity, and enhancement of productivity of suspension feeders. The data also suggest that deposition of crinoidal strata ceased in water shallower than 23 m and then was followed by cementation, pelagic sedimentation, ferromanganese crust, and unconformity. Because it is well known that crinoid productivity can flourish at depths much greater than 23 m (Roux et al., 1988), its termination in such shallow water must be explained. Such shallow-water extermination 785
of suspension feeders can be explained with the nutrient overload model, but in this case, dysoxia might have been achieved at relatively shallow water depths, killing off benthic suspension feeders. In such a system, the critical interface for crinoid survival may have been shallower than 23 m at times. The shallow zone of oxidation could have fluctuated with variation in wave energy and organic productivity. Crinoid extermination due to eutrophication and oxygen deficiency in such shallow water would have been episodic, because normally, shallow waters should have been in the well-oxidized mixed layer. Given this scenario, one would predict that there should be other evidence for at least ephemeral lowered oxygen conditions and burial of organic matter in deep-water deposits of the same age. In an adjacent basin, such deposits are preserved as thin layers of black shale interstratified with grayish wackestone and packstone containing radiolarians and sponge spicules (Marineo basin, Di Stefano et al., 2002). With this scenario, one would also predict some reduction of pore fluids in shallow water. Assuming an open system of pore fluids, bright cathodoluminescence of some of the submarine cements studied here is consistent with elevated Mn21 and ephemeral moderate decrease in Eh of pore fluids (Barnaby and Rimstidt, 1989) in water depths between 23 and 112 m. Given this mechanism for platform termination, one would expect that such an environmental perturbation would be more widespread than just one platform. Similar Pliensbachian productivity perturbations are indicated in carbonate platforms of the central-northern Apennines (central Italy) and of the High Atlas (Morocco) (Galluzzo and Santantonio, 2002; Blomeier and Reijmer, 1999). In the Apennines these perturbations are correlated to a positive peak in marine d13C from adjacent coeval basinal deposits (Galluzzo and Santantonio, 2002). These observations suggest that the causes for eutrophication are more widespread than local phenomena. CONCLUSIONS 1. A fluid-inclusion geobarometer is useful for quantifying water depth of early diagenesis in ancient carbonate platforms. 2. In this application, crinoidal carbonates, a facies notoriously difficult to ascribe to a particular water depth, formed in water depths less than 23 m. 3. The quantification of paleo–water depth shows that during a transition to Pliensbachian platform termination, maximum rate of relative rise in sea level was only 7 m/m.y., well
786
below the potential for carbonate productivity in a healthy shallow-water tropical system. 4. Slow rates of relative rise in sea level indicate that the transition to platform termination was due to an environmental perturbation negatively affecting carbonate productivity. The transition to dominance of suspension feeders, in water depths less than 23 m, indicates a nutrient-overloaded system. 5. The complete termination of benthic production took place, in water shallower than 23 m, because of eutrophication and short-lived dysoxia. 6. The initial deposition of pelagicdominated rosso ammonitico facies likely took place in water deeper than 112 m. ACKNOWLEDGMENTS We thank Paul Enos, Wolfgang Schlager, and J.F. Read for their critical reading of the manuscript and helpful comments. This study was supported by MURST-PRIN 2001 (grant to Di Stefano). REFERENCES CITED Barnaby, R.J., and Rimstidt, J.D., 1989, Redox conditions of calcite cementation interpreted from Mn and Fe contents of authigenic calcites: Geological Society of America Bulletin, v. 101, p. 795–804. Bice, D.M., and Stewart, K.G., 1990, The formation and drowning of isolated carbonate seamounts: Tectonic and ecological controls in the northern Apennines, in Tucker, M.E., et al., eds., Carbonate platforms, facies, sequence and evolution: International Association of Sedimentologists Special Publication 9, p. 145–168. Blomeier, D.P.G., and Reijmer, J.J.G., 1999, Drowning of a Lower Jurassic carbonate platform: Jbel Bou Dahar, High Atlas, Morocco: Facies, v. 41, p. 81–110. Cobianchi, M., and Picotti, V., 2001, Sedimentary and biological response to sea-level and palaeoceanographic changes of a Lower-Middle Jurassic Tethyan platform margin (Southern Alps, Italy): Palaeogeography, Palaeoclimatology, Palaeoecology, v. 169, p. 219–244. Di Stefano, P., Gala´cz, A., Mallarino, G., Mindszenty, A., and Vo¨ro¨s, A., 2002, Birth and early evolution of a Jurassic escarpment: Monte Kumeta, western Sicily: Facies, v. 46, p. 273–298. Dominguez, L.L., Mullins, H.T., and Hine, A.C., 1988, Cat Island platform, Bahamas: An incipiently drowned Holocene carbonate shelf: Sedimentology, v. 35, p. 805–819. Farinacci, A., Mariotti, N., Nicosia, U., Pallini, G., and Schiavinotto, F., 1981, Jurassic sediments in the umbro-marchean Apennines: An alternative model, in Rosso Ammonitico Symposium Proceedings: Rome, Italy, Edizioni Tecnoscienza, p. 335–398. Galluzzo, F., and Santantonio, M., 2002, The Sabina Plateau: A new element in the Mesozoic paleogeography of Central Apennines: Bollettino della Societa` Geologica Italiana Special Paper 1 (in press). Garrison, R.E., and Fischer, A.G., 1969, Deep-water limestones and radiolarites of the alpine Jurassic, in Friedman, G.M., ed., Depositional environments in carbonate rocks: Society for Sedimentary Geology Special Paper 14, p. 20–56. Goldstein, R.H., and Franseen, E.K., 1995, Pinning points: A method providing quantitative con-
straints on relative sea-level history: Sedimentary Geology, v. 94, p. 1–10. Goldstein, R.H., and Reynolds, T.J., 1994, Systematics of fluid inclusions in diagenetic minerals: Society for Sedimentary Geology Short Course Notes 31, 199 p. Gradstein, F.M., Agtemberg, F.P., Ogg, J.G., Hardenbol, J., Van Veen, P., Thierry, J., and Huang, Z., 1994, A Mesozoic time scale: Journal of Geophysical Research, v. 99, no. B12, p. 24 051–24 074. Hallock, P., and Schlager, W., 1986, Nutrient excess and the demise of coral reefs and carbonate platforms: Palaios, v. 1, p. 389–398. Hallock, P., Hine, A.C., Vargo, G.A., Elrod, J.A., and Jaap, W.C., 1988, Platform of the Nicaragua Rise: Examples of the sensitivity of carbonate sedimentation to excess trophic resources: Geology, v. 16, p. 1104–1107. James, N.P., and Choquette, P.W., 1990, Limestone: Introduction, in McIlreath, I.A., and Morrow, D.W., eds., Diagenesis: St. John’s, Newfoundland, Geoscience Canada, p. 9–34. Johnson, W.J., and Goldstein, R.H., 1993, Cambrian sea water preserved as inclusions in marine low-magnesium calcite cement: Nature, v. 362, p. 335–336. Newell, D.K., and Goldstein, R.H., 1999, A new technique for surface and shallow subsurface paleobarometry using fluid inclusions: An example from the Upper Ordovician Viola Formation, Kansas, USA: Chemical Geology, v. 154, p. 97–111. Pa´lfy, J., Smith, P.L., and Mortensen, J.K., 2000, A revised numeric time scale for the Jurassic, in Hall, R.L., and Smith, P.L., eds., Advances in Jurassic Research 2000: Zu¨rich, TransTech, p. 181–182. Roedder, E., 1970, Application of an improved crushing stage to study gases in fluid inclusions: Schweizerische Mineralogische und Petrographische Mitteilungen, v. 50, p. 41–58. Roux, M., Bourseau, J.P., Bas, T., Dumont, T., de Graciansky, P.C., Lemoine, M., and Rudkiewicz, J.L., 1988, Bathymetric evolution of the Tethyan margin in the western Alps (data from stalked crinoids): A reappraisal of eustatism problems during the Jurassic: Bulletin de la Socie´te´ Ge´ologique de France, v. 4, p. 633–641. Schlager, W., 1981, The paradox of drowned reefs and carbonate platforms: Geological Society of America Bulletin, v. 106, p. 1143–1157. Schlager, W., 1999, Scaling of sedimentation rates and drowning of reefs and carbonate platforms: Geology, v. 27, p. 183–186. Winterer, E.L., 1998, Paleobathymetry of Mediterranean Tethyan Jurassic pelagic sediments: Memorie della Societa` Geologica Italiana, v. 53, p. 97–131. Winterer, E.L., and Bosellini, A., 1981, Subsidence and sedimentation on Jurassic passive continental margin, Southern Alps, Italy: American Association of Petroleum Geologists Bulletin, v. 65, p. 394–421. Zempolich, W.G., 1993, The drowning succession in Jurassic carbonates of the Venetian Alps, Italy: A record of supercontinental breakup, gradual eustatic rise, and eutrophication of shallow-water environments, in Loucks, R.G., and Sarg, J.F., eds., Carbonate sequence stratigraphy: American Association of Petroleum Geologists Memoir 57, p. 63–105. Manuscript received January 30, 2002 Revised manuscript received May 1, 2002 Manuscript accepted May 3, 2002 Printed in USA
GEOLOGY, September 2002
