Since salt deposition, the basin has filled principally by prograding sands and muds and, to a 
lesser degree, by transgressive carbonates. The Triassic to modem sediments vary from less than 3 
feet to as much as 50,000 feet in thickness (2). Fluvial and upper deltaic plain sands and muds 
compose the thinner onshore (updip) part of the sedimentary sequence; deltaic sands and muds and 
organic-rich slope muds with fine sands and silts form the thickest part of the section. Distal-slope 
and abyssal muds rapidly thin in the far offshore part of the basin fill. 
Overlying the salt, Jurassic and lower Cretaceous continental sediments filled the salt withdrawal 
basins. Superimposed Cretaceous deposits are dominated by shelf-edge carbonate systems of reefal 
and bank origins that commonly grade into calcareous, organic, very fine grained slope sediments. 
Cenozoic facies are dominated by overlapping, progradational sediments similar to those now being 
deposited by the Mississippi River on its delta and adjacent shelf and slope. The combination of salt 
gliding under loading, salt diapirism, salt withdrawal from basins and associated faulting, and low- 
angle, down-to-the-coast growth faulting characterizes the deep geologic structure of the north¬ 
western Gulf Coast Basin. Figure 2 is a representative cross section illustrating the stratigraphy and 
structure near the study area (3). 
The Gulf Coast Basin is a rich petroleum province, and numerous oil and gas fields produce from 
traps underlying the Galveston Bay System and adjacent onshore properties. A major oil and gas 
play, the deep-seated Frio salt dome play (4), occurs in an area of deeply buried salt diapirs sur¬ 
rounded by shallow piercement domes that formed contemporaneously with the Frio-age (Oligo- 
cene) Houston delta system. Cedar Point and Trinity oil fields underlie the bay and have produced, 
respectively, 13.2 and 21.2 million barrels of oil. On the west side of Galveston Bay, Clear Lake (22.1 
mmbbl), Gillock (24.4 mmbbl). South Gillock (20.7 mmbbl). East Gillock (44.3 mmbbl), and Webster 
(528.0 mmbbl) are onshore fields producing from the same play. In addition, many other productive 
fields occur in smaller plays containing sandstone reservoirs formed in progradational sequences, 
faulted zones and deformed strata surrounding salt diapirs. 
Surficial Geology 
The surficial deposits surrounding the Galveston Bay System represent only recent geologic 
history, the final depositional and erosional phases of the Pleistocene ice ages, and the Holocene post¬ 
glacial events (5) (Figure 3). The major control effecting most geomorphic features and sedimentary 
deposits is the recent history of sea-level fluctuations. Sea level was lowered by nearly 450 feet when 
glaciers advanced to their farthest limits on the northern continents. Then streams like the San Jacinto 
and Trinity Rivers eroded deep broad valleys entrenched into the land and former continental 
shelves and deposited their sedimentary loads onto the former shelf and slope. Sea level was highest 
when the glaciers melted and retreated. A rising sea inundated the entrenched valleys, and the locus 
where the streams deposited their sediments progressively shifted landward. All the modem sed¬ 
imentary systems owe their attributes to the most recent sea-level rise following the last major glacial 
advance in North America. The size and shape of bays, inlets and barrier islands reflects this most 
recent eustatic cycle. 
Two Pleistocene formations, the Beaumont and Deweyville Formations, crop out near Galveston 
Bay. The Beaumont Formation is composed predominantly of clay, silt and sand where the sediments 
were deposited in fluvial, delta plain and bay environments. A large river system, having meander 
channels larger than those of today, transported mainly sand and silt when sea level was lowered 
during glaciation and while sea level rose during interglacial periods. An extensive Beaumont deltaic 
plain is composed of sand and silt deposited in the distributary channels and of organic-rich clays 
and silts deposited in the interdistributary areas. Locally, fine-grained and fossiliferous muds 
represent former bay deposits. Some Beaumont sediments are composed of mostly fine-grained sand 
arranged in linear trends parallel to the coast. These linear features are higher in elevation (>8 feet) 
than surrounding sediments, and they are characterized by pimple mounds and circular depres¬ 
sions. These sand-rich deposits represent a former barrier island much like those of the modem Gulf 
Coast. 
The Deweyville Formation, which is generally younger than the Beaumont Formation, contains 
coarser grained sediments including gravel. These fluvial-dominated sandy sediments rarely con¬ 
tain clay and silt except in outcrops of backswamp facies. Deweyville exposures, which also exhibit 
7 
