Marine offshore to alluvial plain transitions within the "Chemung"-Hamsphire interval (Upper Devonian) of the southern central Appalachians

Abstract

The Upper Devonian "Chemung" and Hampshire Formations of the southern Central Appalachian basin constitute the upper strata of an eastward thickening wedge of elastic sediment. The sediments comprising the "Chemung" and Hampshire, deposited on a low-gradient marine shelf and alluvial plain, respectively, are syn- to post-orogenic (Acadian), and were derived from the erosion of the Acadian Mountains to the east.

Facies of the "Chemung" indicate deposition within three major, laterally adjacent environments. The predominating facies assemblage constitutes upward-coarsening sequences interpreted as offshore to upper shoreface progradational successions. The sequences are storm- and wave-dominated and pass upward into storm- and wave-dominated shoal water delta facies or into low-energy subtidal to supratidal mudflat facies. The absence of beaches and tidal channels is indicative not only of low tidal ranges, but also of seaward dissipation of wave-energy within a very shallow intracratonic foreland basin.

The easternmost sections of the Hampshire Formation are comprised of fluvial cycles interpreted as high-energy sheetflood deposits.This style of sedimentation developed in closest proximity to the source. The Hampshire of more western sections is constructed predominantly of cycles interpreted as low-sinuosity braided stream deposits with a lesser abundance of cycles deposited by high-sinuosity meandering streams and high-energy sheetfloods. Low depositional slopes were associated with the alluvial plain. Braiding, however, was probably due to high runoff associated with high rainfall in a tropical climate and inadequate floodplain stabilization by vegetation, resulting in periodic high discharge.