256-4 Where is the sequence boundary in Pennsylvanian cyclothems?: Unraveling a quiet, critical debate with implications for evolutionary paleoecology

Session: Life and Environments Through Time and Space: Multi-Record Approaches to Stratigraphic Paleobiology, Part I

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Abstract:

Pennsylvanian cyclothems are 4^th and 5^th order transgressive-regressive sequences deposited in the paleotropics that preserve a record of repeated environmental change in response to Milankovitch-driven glacial-eustatic cycles. Interpreting this record requires correct placement of cyclothem sequence boundaries and a better understanding of terrestrial paralic environments.

In most cyclothems with terrestrial sediments, marine facies overlie coal, which overlies a paleosol. Traditionally, the cycle boundary lies at the top of the coal, placing the paleosol and coal at the end of the preceding cycle. However, cyclothem paleosols and coal also reveal changes in paleotropical climate tied to Pennsylvanian glacial eustacy. In cyclothems, vertisols, which indicate seasonally dry tropical climates, underlie coals, which indicate extremely wet, perhumid climates. Today, peat accumulation in tropical swamps requires a continuously rainy climate (no more than 4 consecutive weeks without rain). Vertisols overlain by coal indicate that Pennsylvanian Milankovitch cyclicity drove paleotropical climate cycles, from seasonally dry to very rainy. A sequence boundary at the top of the coal would place the rainy interval of coal accumulation at the time of maximum glacial advance.

If peat accumulation on coastal plains is an early response to rising sea level, the cyclothem sequence boundary lies at the top of the paleosol, relegating seasonally dry paleosols to the upper, regressive part of the previous cyclothem, during the time of maximum glacial advance. Placing the sequence boundary below the base of the coal implies that paleotropical climate became wetter as glaciers melted.

The carbonate geochemistry of coal balls, which are carbonate concretions found sporadically in Late Paleozoic paleotropical coals, supports this interpretation. Coal-ball formation was a two-step process. First, high-Mg calcite or dolomite, indicative of brackish/marine pore water, cemented large pore spaces in the peat. Next, ferroan, low-Mg calcite indicative of fresh pore water permineralized the plant cell walls, placing coal balls formation in the marine-freshwater mixing zone of coastal swamps. Pennsylvanian paleobotanists have resisted the hypothesis that coal-ball plants grew in salt or brackish water. However, the exquisite preservation of delicate roots in coal balls suggests that these plants grew in marine swamps, like modern mangroves. Although most Pennsylvanian coal accumulated in fresh water, coal with coal-balls accumulated in marine/brackish swamps, supporting placement of the sequence boundary at the base of the coal.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7708

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