Characteristics and Evolution of Paleovalley Systems in Settings Where Accommodation Decreases Down Dip

Abstract

Recent advances in our understanding of the morpholological evolution of paleovalleys, the composite nature of the associated sequence boundary and its expression in down-dip locations have been largely guided by examples from passive margins, where subsidence increases basinward. This study documents a series of incisional fluvio-deltaic sandbodies from the Pennsylvanian Breathitt Group (central Appalachian Basin, USA), which fulfil the traditional definition of paleovalley fills. The Breathitt Group was deposited in an epicontinental foreland basin setting, in which there was no shelf-slope break, and paleovalleys can be tracked from the high accommodation orogenic margin towards the lower accommodation cratonic margin of the basin, over 100 km down-dip. Thus the upper Breathitt Group provides the opportunity to describe changes in up-dip to down-dip characteristics of paleovalleys from setting that contrasts markedly with continental shelf margins. Sandbody architecture has been captured through a combination of centimetre-scale sedimentary logging and annotation of photomosaics from km-long road-cuts to produce correlation panels. Sandbodies are typically 5–20 m thick, 0.5–30 km wide, and dominated by trough cross-bedded medium-to-coarse grained sandstone deposited as longitudinal bars. Heterolithic strata displaying lateral accretion occur, particularly towards the tops of valley fills, as well as rarer heterolith-filled abandonment plugs, slumps and slides. Characteristic changes between the proximal, high accommodation and the distal, low accommodation sectors of the basin include: (1) the number of stratigraphic levels containing major sandbodies decreases, and the sandbodies become increasingly discontinuous, suggesting an overall distributive morphology; (2) the sandbodies erode into increasingly open-marine facies; (3) the sandbodies contain an increasingly diverse, marine ichnofauna, suggesting increasing marine influence down depositional dip. Up-dip, a basinward facies shift at the bases of the paleovalleys is not evident, whereas down-dip an unambiguous basinward facies shift at the bases of the same sand bodies clearly distinguishes them as paleovalley fills. This contrasts with models for paleovalleys derived from passive margins, where the expression of the paleovalley is lost down-depositional dip, and poses the question “where does a channel complex become a palaeovalley-fill?” in settings where accommodation decreases down-dip.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015