CARBONATE CYCLICITY IN CORRELATIVE MIDDLE- AND OUTER-SHELF SECTIONS OF DEMOPOLIS CHALK (EASTERN GULF COASTAL PLAIN, U.S.A): EVIDENCE FOR CLIMATE-DRIVEN CLASTIC DILUTION/ORGANIC PRODUCTIVITY CYCLES, SEDIMENTATION RATES, AND IMPLICATIONS FOR ORGANIC MATTER ACCUMULATION

The Upper Cretaceous Demopolis Chalk, eastern U.S. Gulf coastal plain, is characterized by decimeter-scale alternation of chalks, marly chalks, marlstones, and calcareous mudstones that reflects depositional cycles linked to axial precession mediated by orbital eccentricity. Comparative analysis of relatively clastic-rich middle-shelf and relatively clastic-poor outer-shelf sections of the Demopolis Chalk provides evidence for a climatic/oceanographic driver of depositional cyclicity and a means of estimating relative and absolute rates of accumulation of carbonate, clastic detritus, and organic matter. Variability of carbonate contents and carbonate-cycle amplitudes between correlative intervals of middle- and outer-shelf sections, in conjunction with organic carbon and carbonate ẟ¹³C data, demonstrate that Demopolis depositional rhythms mainly reflect cyclic variations in riverine influx of clastic sediments and nutrient-rich waters—i.e., they record clastic dilution/organic productivity cycles, presumably in response to wet/dry climate cycles. Based on the precession cycle geochronometer and assuming essentially constant carbonate production, total, clastic, and organic accumulation rates for the middle shelf site ranged from 3-10.5 cm/kyr (average 3.85 cm/kyr), 0.4-7.5 cm/kyr (1.27 cm/kyr), and 0.01-0.11 cm/kyr (average 0.035 cm/kyr), respectively. Based on comparison of correlative intervals, these rates were on average ~1.2x, 2.4x, and 1.5x higher than at the outer-shelf site. Organic accumulation rates increased progressively with clastic accumulation rates, reflecting concomitant increases in productivity and burial-related preservation. However, despite higher organic accumulation rates, evidence indicates that TOC contents leveled off or decreased due to dilution when clastic accumulation rates reached threshold levels of ≥3 cm/kyr.