ABSTRACT

The Glen Rose formation (Lower Cretaceous) contains a microfossil flora that is useful as a paleoecological tool. Examination of measured sections and three mapped terrigenous-clastic units shows that plant microfossils, especially gymnosperm pollen and dinoflagellate hystrichospheres, are particularly useful in diagnosis of subtidal and supratidal conditions when coupled with lithologic evidence. Other microfloral elements such as fern spores and angiosperm pollen were of limited use in paleoenvironmental intepretations.

Contoured percentage distributions of gymnosperm pollen and hystrichospheres across three units in a terrigenous clastic sequence in the Glen Rose of Somervell County reveal an increase of hystrichospheres and a decrease in gymnosperms on moving toward probable open water sediments. This suggests that palynomorph number is strongly influenced by distance from source area.

Hystrichosphere morphologic types were found to vary with depositional environment. In several instances barb spined forms were associated with sediments of shallow water open marine origin. Straight spined forms were commonly associated with sediments interpted as representing shallow brackish water conditions. Non-spinose forms were formed in both environments.

Some microfossils, such as angiosperms, fern spores, and various hystrichosphere types were found to be not clearly related to depositional environment as inferred from lithologic evidence and other palynomorphs. The distribution of these microfossils may likely be explained by wind and water current fluctuations during their deposition. Particle characteristics such as settling velocity and size sorting of individual palynomorphs may have also strongly influenced their distribution.