East Texas (Brazos/Colorado)
Highstand Systems Tract
The HST of the east Texas: Brazos/Colorado region is separated into two distinct stages (early and late) based on depositional style. During the early stage the bedload-dominated Colorado River formed a lobate sandy delta. In contrast, the suspended load-dominated Brazos River deposited an elongate muddy delta. Three phases of growth, followed by backstepping events, are recognized within the Brazos delta and these have tentatively correlated to fith-order eustatic fluctuations during Stage 5 (Abdulah, 1995; 1996 ISG Report). Seismic facies and platform boring descriptions show that these elongate delta lobes are comprised dominantly of silt and clay.



During the late highstand, sea-level fell to an outer shelf location, except for a brief period during Stage 3. During this time interval, the Brazos and Colorado rivers constructed more typical birds foot-type, fluvial-dominated deltas with branching distributaries and distributary mouth bars. These younger deltas have onlapping relationships with the older early highstand deltas. There are no platform borings through these distributary channels and mouth bar facies, but their seismic character implies an abundance of sand. We believe that the dramatic change in the style of delta evolution may have resulted from a climatic change (increased precipitation and associated fluvial and sediment discharge) in the drainage basins of these rivers, which includes virtually all of west Texas. It is also probable that the rate of sea-level fall decreased during the time these fluvial-dominated deltas formed (the Stage 2 to Stage 3 transition, although better chronostratigraphic control is required to document this change. The topset beds of the late highstand deltas have been removed by ravinement processes from a subsequent rise in sea level.

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