Transverse Palaeovalleys Pass Up-Dip Into Conformable Strata in a Foreland Basin Setting

Abstract

Recent advances in our understanding of palaeovalleys are largely guided by examples from passive margins. This study tests these models against a dataset from the Pennsylvanian Breathitt Group of the central Appalachian foreland basin, USA. This fluvio-deltaic succession contains extensive erosionally-based sandbodies that can be tracked transversely from the high accommodation orogenic margin to the low accommodation cratonic margin of the basin, over 100 km down depositional dip. The succession is exposed along numerous road cut exposures, 90 of which were targeted for study. More than 4 km of centimetre-scale sedimentary logs were collected, and vertical and lateral facies relationships were captured through the annotation of photomosaics. Sandbodies are typically 5–40 m thick, 0.5–30 km wide, and dominated by fluvial to estuarine trough cross-bedded medium-to-coarse grained sandstone deposited as longitudinal bars. Heterolithic strata displaying lateral accretion, slumps and slides also commonly occur within the sandbodies. Two major sandbody types are distinguished. The first exhibit a down-dip decrease in the number of storeys and individual storey height, do not exhibit a basinward facies shift at their base, and pass down-dip into delta front mouthbar successions. These are interpreted as stacked distributary channels. The second type shows little change in the number of storeys and individual storey height down depositional dip. Up-dip, these sandbodies display a conformable relationship with the strata into which they incise (e.g. levees which fine away from the sandbodies), but down-dip these sand bodies erode into increasingly open-marine facies of the delta front and display a marked basinward facies shift at their bases. These are interpreted as palaeovalley fills which pass upward into simple stacked distributaries. Floral assemblages from channel plugs within palaeovalley-fills are enriched in taxa with a physiology better adapted to water-stress, and are consistent with hydraulic draw-down or relative climatic aridity associated with palaeovalley formation. These results support recently developed models based on Quaternary datasets, as well as numerical modelling, which suggest that palaeovalleys should pass laterally into conformable strata. However, in foreland basins, transverse palaeovalleys demonstrably pass up-dip into conformable strata, compared to passive margins in which palaeovalleys pass down-dip into conformable strata.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015