An exhumed fine-grained meandering channel in the lower Permian Clear Fork Formation, north-central Texas: Processes of mud accumulation and the role of vegetation in channel dynamics

Abstract

Ancient fine-grained meandering channels are under-represented in the literature and their formative processes are rarely explored. The Montgomery Ranch 3 site of the Clear Fork Formation of Texas contains an exhumed fine-grained point bar that migrated for at least 50m within a channel 2m deep and 36m wide. The point bar comprises thick inclined layers of unstratified mudstone intercalated with thin layers of fine-grained, ripple cross-laminated sandstone, with dips averaging nearly 16o. Rill casts and swept ripples on the sandstone surfaces indicate declining water levels. Petrographic analysis of the mudstone shows silt and clay laid down from suspension, but sand-sized mud aggregates transported as bedload (present at other sites in the formation) were not observed. The sandstone beds are attributed to lateral accretion on the point bar during periods of sustained flow, whereas the mudstone beds are attributed to oblique accretion as fine sediment draped the bar during waning and low-flow periods. Sandstone and mudstone units are composite units from numerous flow events and their alternation may reflect secular variation in flood frequency and intensity. In an associated abandoned-channel fill, weakly laminated mudstone with desiccation cracks contains leaves and seeds of Evolsonia texana, marattialean foliage and Taeniopteris sp., with root traces penetrating the leaves. Some taxa preferred high water tables and humid conditions, whereas others were dryland colonisers. This apparent discrepancy may reflect the persistence of wetter channel reaches within an otherwise dry setting. Despite the scarcity of preserved plant fossils, vegetation was probably sufficiently widespread to promote bank strength and local sediment accumulation.