Progradational Successions of Distributive Fluvial Systems Affected by Eustatic Forcing: New Model From the Carboniferous Upper Breathitt Group (Kentucky, U.S.A.)

Abstract

Recent advances on the geomorphology and stratigraphy of distributive fluvial systems (DFS) have highlighted their importance for the accumulation of thick alluvial successions in ancient continental records. Much research so far has been conducted on systems aggrading within land-bound interior basins, virtually unaffected by interactions with coastal environments and by sea-level changes. We propose a reinterpretation of the Carboniferous upper Breathitt Group (Central Appalachian Basin, southeast Kentucky) in terms of long-term progradation of contiguous DFSs subject distally to incision during base-level drawdown and transgressive encroachment by marginal-marine environments, paced by late-Paleozoic glacioeustasy.

The Breathitt Group was deposited in a subequatorial foreland basin during the Alleghenian phase of the Appalachian Orogeny; its uppermost formations consist mainly of mudstones and subarkosic sandstones representing late-orogenic clastic debris transported transversely to the basin axis by drainages that terminated along a shallow epicontinental seaway. At basin scale, these formations present: 1) tabular, aggradational geometry (typified by the basin-wide extent of coal zones); 2) numerous fluvio-estuarine incised-valley fills clustered within specific stratigraphic intervals; 3) extensive belts of floodplain facies associations; 4) upward increase in the proportion of terrestrial vs. shallow-marine/paralic deposits; 5) upward increase in the volume, grain size and amalgamation of fluvial channel fills. The alluvial components present sedimentological and architectural trends typical of progradational DFS, whereas associated incised-valley fills and rhythmic alternations of fluvial and shallow-marine facies associations indicate repetitive overprinting by forced-regressive and transgressive events.

The overall architecture of the upper Breathitt Group provides a model for DFS successions affected by high-frequency glacioeustatic sea-level changes of high magnitude in distal basinal settings. With due consideration of differences related to icehouse vs. greenhouse conditions, the concept likely applies to the stratigraphic analysis of other thick ‘fluvio-deltaic’ units worldwide, comprising proximal-to-distal transitions from alluvial to paralic and shallow-marine facies associations, such as the Devonian “Catskill Delta” (northeast USA), parts of the Jurassic Brent Group (North Sea) and of the Cretaceous Mesaverde Group (mid-western USA).

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018