Control of terrestrial stabilization on Late Devonian palustrine carbonate deposition; Catskill Magnafacies, New York, U.S.A.

Upper Devonian (Frasnian) terrestrial strata of south-central New York contain palustrine and lacustrine carbonate deposited within the well-developed Catskill clastic wedge succession. These nonmarine limestone beds (8-50 cm thick) were repeatedly subaerially exposed and subjected to pedogenic processes. Palustrine features include subaerial exposure surfaces with soil crusts, pseudo-microkarst and microkarst, brecciation, desiccation cracks, horizontal, planar, and circumgranular cracks, and rhizoliths. Lacustrine carbonate sedimentation was derived primarily from biogenically induced precipitation and from degradation of calcified charophyte stems and ostracodes. These deposits accumulated in relatively shallow water depths, probably < 10 m. The results of stable isotope analyses (delta ¹³ C = -4.0 to -4.7 o/oo PDB; delta ¹⁸ O from -6.8 to -8.7 o/oo PDB) reveal covariance between delta ¹³ C and delta ¹⁸ O values. The high degree of covariance (r = 0.75) suggests that these Late Devonian carbonate lakes were hydrologically closed; high rates of surface-water productivity resulted in the heavier delta ¹³ C values. Late Devonian carbonate lakes developed as a result of landscape stabilization by the developing rhizosphere. In addition, plants of small to moderate stature functioned as clastic filters, trapped terrigenous clastic sediment along lake margins, and thereby permitted carbonate sedimentation to occur in a system otherwise dominated by terrigenous clastics. These Upper Devonian lacustrine deposits contain the oldest recognized occurrence of "palustrine" facies. The temporal distribution of palustrine carbonate deposits therefore appears limited to post-Silurian strata.