Stable Isotope Tests of Evaporative Marine Environments in ... · The Silurian Period experienced...
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Stable Isotope Tests of Evaporative Marine Environments in the Late Silurian Tonoloway Formation of West Virginia B Chemostratigraphic column of 40 meter section of Tonoloway Formation outcrop. Panel A shows the lithologies and sedimentary textures of the outcrop. Panel C shows carbonate δ 13 C values in blue and δ 18 O values in red. Circles represent micrite values, triangles represent laminite values, and squares represent dolomicrite values. Panel D shows the Total Organic Carbon and Panel F shows Total Sulfur. Deborah Fishbeck Advisor: A. J. Kaufman δ 13 C Carbonate δ 13 C Organic δ 34 S Pyrite δ 18 O Carbonate Micrite Average -3.55 -28.18 13.59 -7.84 Std Dev 1.89 0.99 7.32 1.20 Laminite Average -2.46 -29.07 9.62 -7.67 Std Dev 0.73 1.52 14.64 1.36 Dolomicrite Average -1.80 -28.08 11.28 -5.77 Std Dev 1.10 0.92 6.27 0.39 Intraclastic Brecciation Average -1.97 -27.95 9.36 -6.49 Std Dev 0.67 n/a n/a 1.00 Fossils Average -3.28 -28.10 12.76 -8.38 Std Dev 1.77 0.91 7.35 0.50 Vugs Average -4.74 -28.86 17.31 -7.25 Std Dev 1.45 0.72 5.15 1.32 Mud-cracks Average -4.59 -28.85 18.87 -7.53 Std Dev 1.00 0.44 2.31 1.38 This research project focused on a recently exposed carbonate-rich succession of the Silurian Tonoloway Formation. This unit preserves sedimentary evidence of high degrees of seawater evaporation in the form of mud-cracks, halite-hoppers, and trace gypsum and anhydrite, which conceivably resulted in 18 O and 13 C enrichments in evaporitic facies relative to open marine environments. To test whether isotopic analyses might provide clues to ancient marine carbonates where sedimentary evidence for evaporation is lacking, I conducted a field and laboratory-based study of the Tonoloway Formation, including the construction of a stratigraphic column and collection of a suite of 43 samples for detailed petrographic and isotopic analysis. I hypothesize that strata containing textures indicative of an evaporitic and saline depositional environment will be enriched in 13 C and 18 O compared to non-evaporative facies, and that 13 C and 34 S abundances of organic matter and pyrite, which are biologically controlled, may also show anomalous enrichments due to environmental stress. The Upper Silurian Tonoloway Formation contains a range of shallow marine to terrestrial near shore facies, and the outcrop of study is made primarily of subtidal, intertidal, and supratidal facies. The Tonoloway Formation is made of shallowing upward depositional packages, capped with intertidal and supratidal facies, deposited in a arid sabkha flat (Bell,1997). The Silurian Period experienced four major 13 C isotope excursions. The Tonoloway was deposited in the isotopic calm of the Pridoli Stage, between two major 13 C events. Micrite, laminite and dolomicrite were the main lithologies of the outcrop. The upper 15 meters suggest a more evaporative environment due to abundance of supratidal facies. Dolomicrite facies are enriched in carbonate 18 O and 13 C relative to micrite facies by about 2‰ and 3‰. This suggests that dolomitization was related to the evaporative concentration of Mg in seawater through the precipitation of Ca-bearing carbonates and evaporites. 13 C TOC values show no trend or enrichment for any texture or lithology. The percent of Total Organic Carbon is low in all samples. Anomalous 34 S enrichments likely reflect removal of sulfate from the water through pyrite burial and sulfate evaporate mineral precipitation. Chemical analysis of additional Tonoloway outcrops, such as the type section in Pinto Maryland, will provide a greater breadth of chemical signals. Correlation of this chemostratigraphic column to other columns will provide a bigger picture of regional differences in chemical signals of evaporative environments. Petrographic and X-ray analysis will provide additional information. An evaporative model of dolomitization explains high δ 13 C and δ 18 O values. As seawater evaporates on a supratidal flat, the remaining sea water around the sediments are progressively concentrated in heavier isotopes of δ 18 O and δ 13 C, which leaves the forming dolomicrite to be enriched in those heavy isotopes. 34 S enrichments could be due in part to sulfate mineral precipitation in an evaporative depositional environment. An evaporating body of seawater resolves the kinetic barriers to dolomitization: an increase in temperature, a freshening of hypersaline waters, and an increase of seawater Mg/Ca ratio to about 0.8. • Evaporating shallow water is warmer in temperature than deeper water. • Seawater can be freshened with the influence of meteoric waters. • The Mg/Ca ratio of about 0.8 can achieved through removal of calcium in the form of precipitating gypsum and calcite. Seawater isotopic composition of δ 13 C from Middle Cambrian to Pennsylvanian Period. The Silurian is highlighted in red, and the Pridoli is marked with a star. (Saltzman, 2002) Schematic of subtidal, intertidal and supratidal zones. Micrite is deposited in subtidal zone, laminate is deposited in intertidal zone, and dolomicrite is supratidal. Outcrop along highway corridor H. Red dots mark sample collection site. Image of vugs and halite hoppers, which indicate evaporitic marine depositional environment. Needle width of 250 mm. Intraclastic breccias from supratidal environments reveal the greatest enrichment in 13 C and 18 O abundances. Dolomicrite is the lithology that shows greatest enrichment in 13 C and 18 O abundances. Saltzman, M., 2002. Carbon isotope (δ13C) stratigraphy across the Silurian–Devonian transition in North America: Evidence for a perturbation of the global carbon cycle. Palaeogeography, Palaeoclimatology, Palaeoecology, 187(1-2): 83-100. Bell, S. 1997. Cyclic lithofacies, sequence stratigraphy, and depositional basin evolution of the Tonoloway Limestone [dissertation]. [Morgantown (WV)]: West Virginia University. Vugs and mud-cracks do not show enrichment in 13 C and 18 O abundances.
Transcript of Stable Isotope Tests of Evaporative Marine Environments in ... · The Silurian Period experienced...
Stable Isotope Tests of Evaporative Marine Environments in the Late Silurian Tonoloway Formation of West Virginia
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Chemostratigraphic column of 40 meter section of Tonoloway Formation outcrop. Panel A shows the lithologies and sedimentary textures of the outcrop. Panel C shows carbonate δ13C values in blue and δ18O values in red.
Circles represent micrite values, triangles represent laminite values, and squares represent dolomicrite values. Panel D shows the Total Organic Carbon and Panel F shows Total Sulfur.
Deborah FishbeckAdvisor: A. J. Kaufman
δ13C
Carbonate
δ13C
Organic
δ34SPyrite
δ18O
Carbonate
MicriteAverage -3.55 -28.18 13.59 -7.84
Std Dev 1.89 0.99 7.32 1.20
LaminiteAverage -2.46 -29.07 9.62 -7.67
Std Dev 0.73 1.52 14.64 1.36
DolomicriteAverage -1.80 -28.08 11.28 -5.77
Std Dev 1.10 0.92 6.27 0.39
Intraclastic Brecciation
Average -1.97 -27.95 9.36 -6.49
Std Dev 0.67 n/a n/a 1.00
Fossils Average -3.28 -28.10 12.76 -8.38
Std Dev 1.77 0.91 7.35 0.50
Vugs Average -4.74 -28.86 17.31 -7.25
Std Dev 1.45 0.72 5.15 1.32
Mud-cracks Average -4.59 -28.85 18.87 -7.53
Std Dev 1.00 0.44 2.31 1.38
This research project focused on a recently exposed carbonate-richsuccession of the Silurian Tonoloway Formation. This unit preservessedimentary evidence of high degrees of seawater evaporation in the form ofmud-cracks, halite-hoppers, and trace gypsum and anhydrite, whichconceivably resulted in 18O and 13C enrichments in evaporitic facies relative toopen marine environments. To test whether isotopic analyses might provideclues to ancient marine carbonates where sedimentary evidence forevaporation is lacking, I conducted a field and laboratory-based study of theTonoloway Formation, including the construction of a stratigraphic columnand collection of a suite of 43 samples for detailed petrographic and isotopicanalysis.
I hypothesize that strata containing textures indicative of an evaporiticand saline depositional environment will be enriched in 13C and 18Ocompared to non-evaporative facies, and that 13C and 34S abundances oforganic matter and pyrite, which are biologically controlled, may also showanomalous enrichments due to environmental stress.
The Upper Silurian Tonoloway Formation contains a range of shallowmarine to terrestrial near shore facies, and the outcrop of study is madeprimarily of subtidal, intertidal, and supratidal facies. The TonolowayFormation is made of shallowing upward depositional packages, cappedwith intertidal and supratidal facies, deposited in a arid sabkha flat(Bell,1997).
The Silurian Period experienced four major 13C isotope excursions.The Tonoloway was deposited in the isotopic calm of the Pridoli Stage,between two major 13C events.
Micrite, laminite and dolomicrite were the main lithologies of theoutcrop. The upper 15 meters suggest a more evaporativeenvironment due to abundance of supratidal facies.
Dolomicrite facies are enriched in carbonate 18O and 13C relative tomicrite facies by about 2‰ and 3‰. This suggests that dolomitizationwas related to the evaporative concentration of Mg in seawaterthrough the precipitation of Ca-bearing carbonates and evaporites.
13CTOC values show no trend or enrichment for any texture or
lithology. The percent of Total Organic Carbon is low in all samples.
Anomalous 34S enrichments likely reflect removal of sulfate from thewater through pyrite burial and sulfate evaporate mineralprecipitation.
Chemical analysis of additional Tonoloway outcrops, such as the type sectionin Pinto Maryland, will provide a greater breadth of chemical signals. Correlationof this chemostratigraphic column to other columns will provide a bigger pictureof regional differences in chemical signals of evaporative environments.Petrographic and X-ray analysis will provide additional information.
An evaporative model of dolomitization explains high δ13C and δ18Ovalues. As seawater evaporates on a supratidal flat, the remaining seawater around the sediments are progressively concentrated in heavierisotopes of δ 18O and δ 13C, which leaves the forming dolomicrite to beenriched in those heavy isotopes. 34S enrichments could be due in part tosulfate mineral precipitation in an evaporative depositional environment.
An evaporating body of seawater resolves the kinetic barriers todolomitization: an increase in temperature, a freshening of hypersalinewaters, and an increase of seawater Mg/Ca ratio to about 0.8.
• Evaporating shallow water is warmer in temperature than deeper water.• Seawater can be freshened with the influence of meteoric waters.• The Mg/Ca ratio of about 0.8 can achieved through removal of calcium in
the form of precipitating gypsum and calcite.
Seawater isotopic composition of δ13C from Middle Cambrian to Pennsylvanian Period. The Silurian is highlighted in red, and the Pridoli is marked with a star. (Saltzman, 2002)
Schematic of subtidal, intertidal and supratidal zones. Micrite is deposited in subtidal zone, laminate is deposited in intertidal zone, and dolomicrite is supratidal.
Outcrop along highway corridor H. Red dots mark sample collection site.
Image of vugs and halite hoppers, which indicate evaporitic marine depositional environment. Needle width of 250 mm.
Intraclastic breccias from supratidal
environments reveal the greatest
enrichment in 13C and 18O abundances.
Dolomicrite is the lithology that shows greatest enrichment
in 13C and 18O abundances.
Saltzman, M., 2002. Carbon isotope (δ13C) stratigraphy across the Silurian–Devonian transition in North America: Evidence for a
perturbation of the global carbon cycle. Palaeogeography, Palaeoclimatology, Palaeoecology, 187(1-2): 83-100.
Bell, S. 1997. Cyclic lithofacies, sequence stratigraphy, and depositional basin evolution of the Tonoloway Limestone [dissertation]. [Morgantown (WV)]: West Virginia University.
Vugs and mud-cracks do not show
enrichment in 13C and 18O abundances.
