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Tertiary sediments of the Burgos basin of northeastern Mexico are broken abundantly by faults which are of two types, namely, growth and postdepositional. The growth faults have a sinuous, north-south trend, and are many kilometers in length. Collectively, they make up a series of subparallel blocks of sediments which are faulted in a down-to-basin direction. Although these sediments thicken basinward, they also commonly exhibit rollover (reverse drag) on the downthrown side. In addition, they commonly thicken locally in the opposite direction in close proximity to the growth faults. The structural closures on the downthrown side of such faults are asymmetric and increase in amplitude with depth. Vertical profiles of these faults describe hyperbolic curves which tend to f atten with depth.
Bruce pointed out that such faults in the Texas coastal area "flatten and converge at depth to planes related to fluid pressure and form the seaward flanks of underlying shale masses." He also stated that such "faults formed during time of shoreline regression were developed primarily through differential compaction of adjacent sedimentary masses. These faults die out at depth near the depoaxes of the sandshale section."
Postdepositional faults are more numerous than the growth faults, more closely spaced, and generally have less throw. Their distinguishing characteristic is that the sedimentary section on either side of a postdepositional fault is of the same thickness, whereas the sediments on the downthrown side of a growth fault are always thicker than their counterpart on the upthrown side.
Structural closures on the downthrown side of a growth fault afford ideal conditions for hydrocarbon entrapment. Multiple reservoir sandstones are present. They may consist of one or more crescent to "halo-like" bar sands which were deposited around a growth structure or an isolated sand lens on top of the structure. In rare cases a deltaic-distributary system of sandstones is present in the uppermost part of the stratigraphic sequence of sandstones where the structural closure is minimal.
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