Geological Investigation of Petrified Wood in the Miocene Fleming Formation, Jasper County, Texas

The stratigraphic occurrence of silicified fossil woods in outcrops is documented for a study area in northern Jasper County, Texas, south of the Sam Rayburn Reservoir. This area is located at the northern erosional termination of both the Pliocene Willis Formation and the Miocene Fleming Formation where they overlap the Oligocene to Early Miocene Catahoula Formation.

The upper Catahoula Formation consists of hard, gray to white clayey siltstone and is eroded at the top. It does not contain fossil wood in the study area. The unconformity separating it from overlying formations has many tens of feet of relief over relatively short distances. The Fleming Formation overlies the Catahoula Formation and thins from about 50-70 ft (15-21 m) to wedge-out within the study area but thickens dramatically down-dip. It consists of fine to medium clayey sandstone or sandy claystone with poor sorting and is typically mottled with masses of orange and gray clay, suggesting extensive soil formation on the sediments. The Willis Formation forms an unconformable cap on top of the Fleming Formation and consists of fine to coarse fluvial sandstone that is often iron-stained and cemented.

Petrified wood found in-situ occurs within relatively narrow lenses of fluvial stream channels within the lower 10 ft (3 m) of the Fleming Formation. The wood varies from small pieces to large logs and typically has a small degree of rounding, indicating that it is not in the original fossilization location. However, the presence of large, intact logs indicates that it has not been transported far. Paleobotanical evidence corroborates this, indicating that the logs are either Miocene or Oligocene in origin. This suggests that the petrified wood may have been sourced either in the (now absent) uppermost Catahoula or the lowermost Fleming and that reworking of the sediments exposed and redeposited the fossilized wood.