Carbon isotope excursions and crinoid dissolution at exposure surfaces in carbonates, West Texas, U.S.A.
Carbon isotope excursions and crinoid dissolution at exposure surfaces in carbonates, West Texas, U.S.A.
Journal of Sedimentary Research (April 2006) 76 (3-4): 404-410
- C-13/C-12
- carbon
- carbonate rocks
- Crinoidea
- Crinozoa
- depositional environment
- Echinodermata
- geochemical indicators
- geochemistry
- Invertebrata
- isotope ratios
- isotopes
- Paleozoic
- Permian
- sedimentary rocks
- solubility
- solution
- stable isotopes
- subaerial environment
- terrestrial environment
- Texas
- United States
- West Texas
- Horseshoe Atoll
- Reinecke Field
Carbonates from the Horseshoe Atoll, Reinecke Field, West Texas, U.S.A., show four subaerial exposure events by the presence of rhizoliths, alveolar septal fabric, and thin calcretes that subdivide the Pennsylvanian succession into five sequences. Vertical profiles of bulk rock delta (super 13) C analyses show values that are generally approximately +1 per mil (PDB), but negative shifts of up to 2 per mil occur beneath exposure surfaces. The bulk-rock delta (super 13) C values approximately +1 per mil could be explained as a mixture of 63% marine carbonate (delta (super 13) C = +4 per mil) and 37% pedogenic carbonate (delta (super 13) C = -4 per mil) while a negative bulk-rock excursion of delta (super 13) C = approximately -1 per mil can be explained as a mixture of 37% marine and 63% pedogenic calcite. Distinctive negative shifts in the delta (super 13) C profiles do not occur under some exposure surfaces that contain soil-generated features, and one negative shift occurs that apparently is unrelated to an exposure surface. Bulk-rock delta (super 13) C values are difficult to interpret because the volume of their various components is unknown. Furthermore, delta (super 13) C profiles may miss subaerial exposure surfaces. Certain styles of crinoid alteration may indicate paleo-subaerial exposure surfaces. Fossil crinoids can be preserved in a variety of ways, but crinoids with dissolution holes that subsequently either become filled with calcite cement or were crushed during burial are restricted to 2 m below exposure surfaces. Dissolution of Mg calcite crinoids is due to their relatively high Mg composition, 12 mole % MgCO (sub 3) , combined with their location beneath soil-capped surfaces from which meteoric waters greatly undersaturated with respect to calcite emerged. Late-stage dissolution removed finely crystalline calcite, but crinoids by this time had stabilized to coarsely crystalline calcite and dolomite and thus were unaffected. Evaluation of other stratigraphic systems is needed to determine whether the use of "holey" crinoids to identify subaerial exposure surfaces has broad application.