FINE-GRAINED MEANDERING CHANNELS AND TABULAR SANDSTONES IN THE LOWER PERMIAN CLEAR FORK FORMATION OF NORTH-CENTRAL TEXAS, USA

Although fluvial systems were widespread across Pangea, the range of fluvial styles is poorly understood, due in part to the fine-grained nature of many formations. The Lower Permian Clear Fork Formation is a ~350 m alluvial redbed deposit with fine-grained meandering-channel deposits, tabular sandstones, extensive paleosols, and well-preserved plant and vertebrate fossils. The formation was deposited on the relatively arid western, equatorial margin of Pangea on a stable shelf far from upland source areas, where shallow burial minimized diagenetic effects and preserved original depositional features. The spectrum of fluvial styles has modern analogues on the arid, distal alluvial plains of central Australia.

Three meandering types predominate, with a maximum channel depth of 5 m, steeply dipping lateral-accretion deposits (~15^o), and modest discharge. Quartz-rich systems comprise dunes and ripples that migrated up the lateral-accretion surfaces of sandy point bars. Mud-rich systems comprise ripple cross-laminated mudstone and very fine sandstone, with rare dunes oriented up the inclined beds; sediment was plastered onto accretionary benches and channel banks during oblique accretion. Mud- and quartz-rich systems are exposed as exhumed point bars, comprising thick inclined layers of sand-sized mud aggregates and suspended clays, intercalated with thin layers of ripple cross-laminated quartz-sandstone. This system represents a combination of laterally accreted pedogenic mud aggregates, derived by reworking of adjacent vertic paleosols, and oblique accretion of suspended load. Rill casts, roots and desiccated mudstones at low levels within the channels indicate seasonal flow, and abandoned channels contain finely laminated claystone fills with exquisite plant fossils. Early dolomite and gypsum cements suggest saline groundwater.

The coarse-grained tabular sandstones form extensive sheets up to 8 m thick that cut across channel and floodplain deposits, and contain cross-cutting scours filled with dunes, plane beds, and antidunes. They are interpreted as ephemeral and poorly confined systems that experienced rapid discharge variations under wetter conditions.