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.