FLUVIAL SEDIMENTOLOGY OF THE EARLY PERMIAN CLEAR FORK GROUP OF NORTH-CENTRAL TEXAS: IMPLICATIONS FOR THE PALEOCLIMATE OF EQUATORIAL PANGEA

Historically, Permian paleoclimatic assessment for the equatorial zone of Pangea has relied strongly on evaporitic beds, paleosols and plant fossils. We use here an integrated approach that includes analysis of the alluvial systems to investigate higher order palaeoclimatic trends.

The outcrops of the 400 m thick, Early Permian Clear Fork Group of north-central Texas are world-class sites for plant fossils and contain extensive paleosols that suggest a trend of increased aridity and seasonality of precipitation through the Permian. Through detailed outcrop descriptions and logging, photo-montaging and remote sensing, the stacking patterns of Middle Clear Fork alluvial systems were established for key sites.

Preliminary results suggest that two distinct channel styles are present within these fine-grained systems. Incising into paleosols are fine-grained (fine sand to silt) meandering systems that exhibit steeply inclined heterolithic beds (dips up to 20⁰, average 14⁰), representing lateral accretion of point bars. Channel units are up to 5.9 m thick. Sandy benches and elongate mounds draped by very fine sand and gravel (reworked pedogenic carbonate) are present low in the channel, and bedforms are universally 2D ripples.

Reworked plant fossils are present in the gravel and well preserved fronds were excavated from benches and the lower parts of inclined layers. Rill marks present on the accretionary surfaces within the channel units suggest near-complete flow cessation at times. The channel bodies comprise cross-cutting, laterally accreted lenses, suggesting frequent cutoffs within shallow valleys cut into paleosols.

Laterally extensive sheet sandstones up to 3.4 m thick cap low mesas and cut into the fine-grained channel deposits. These fine- to medium-grained sandstones have straight-crested and climbing ripples, plane beds and trough crossbeds that fill discrete scours. This style suggests ephemeral flashy flow in broad sandbed channels.

These two distinct fluvial styles suggest that climatic conditions during the Early Permian of north-central Texas were relatively arid and strongly seasonal, with near-total drying up of rivers periodically. Although the area was vegetated, the interaction between the vegetation and fluvial systems are poorly understood due to preservation issues.