ABSTRACT

Along the middle and upper Texas coast the updip Yegua Formation (Eocene) has abundant sand in the lower Yegua (Anomalina umbonatus) interval, yet the downdip lower Yegua consists largely of shale in many wells that have drilled this interval. A downdip Jackson County depocenter is an exception with a very thick lower Yegua sand section having been penetrated in a number of wells. An unconformity of basal Yegua age is well developed north or northwest of all studied downdip Yegua sections except the Jackson County depocenter. The steep paleoslopes of this basal Yegua unconformity account for the dearth of expanded lower Yegua sands. This basal Yegua erosional event created a steepened shelf margin/upper slope that resulted in bypassing of lowermost Yegua sands downslope rather than preservation of these sands at the shelf margin. These sands were likely deposited in slope mini-basins and basin floor fans during sea level fall and lowstand.