ABSTRACT

Vertical grain size progressions of sand deposits in the Queen City Formation (Eocene), East Texas, were studied to find textural patterns distinctive of depositional environments. Initially, the environments were interpreted using descriptions of lithology, sedimentary structures, and facies relationships. Then, vertical changes in mean grain size, mode, sorting, skewness, sand content, and frequency weight percent curve shape were used to identify possible definitive grain size trends for each environment. The Queen City Formation was selected because geologists have already delineated, independent of quantitative grain size analysis, numerous depositional environments for this formation. Additionally, the predominance of polycyclic and well sorted fine-grained sand tests the ability of this method to detect subtle changes.

Results from this study indicate that certain groups of environments are more easily distinguished from other groups. Lower point bar, fluvial distributary channel, and distributary mouth bar deposits are more readily differentiated from distal bar, lower shoreface, and upper point bar deposits by their coarser mean grain size and mode, higher sand content, and more prevalent saltation mode and smaller suspension component The lower point bar occasionally has the coarsest mean grain size and mode. Lower shoreface deposits may be distinguished from upper point bar and distal bar sands by their higher sand content and consistent 3.5 suspension mode. Flood-tidal deltaic sands are characterized by their consistent mean grain size, mode, sorting, and predominance of the saltation mode throughout a thick single sequence. Tidal flat sands resemble certain fluvial and fluvial-deltaic sands.

The overlap in grain size traits among many of the environments is attributed to the well sorted and fine-grained nature of the source for the Queen City sands, similarities in transport mechanisms among environments, and, to a lesser degree, diagenetic changes in original grain size distributions.