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Carbon Isotope Excursions and Crinoid Dissolution at Exposure Surfaces in Carbonates, West Texas, U.S.A.

¹ Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, U.K.; jadd1{at}esc.cam.ac.uk
² Unocal, 14141 Southwest Freeway, Sugar Land, Texas 77478, U.S.A.

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 ¹³C analyses show values that are generally ~ +1 (PDB), but negative shifts of up to 2 occur beneath exposure surfaces. The bulk-rock ¹³C values ~ +1 could be explained as a mixture of 63% marine carbonate (¹³C = +4) and 37% pedogenic carbonate (¹³C = –4) while a negative bulk-rock excursion of ¹³C = ~ –1 can be explained as a mixture of 37% marine and 63% pedogenic calcite. Distinctive negative shifts in the ¹³C 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 ¹³C values are difficult to interpret because the volume of their various components is unknown. Furthermore, ¹³C 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 % MgCO₃, 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.