Page 221 - Origin and Prediction of Abnormal Formation Pressures

196 G.V. CHILINGAR, W. FERTL, H. RIEKE AND J.O. ROBERTSON JR.

masses, commonly tens of miles in length, have been observed to range up to 25 miles
(40.23 km) in width and 10,000 ft (3048 m) vertically. Aligned subparallel with the
coast, these features represent residual masses of undercompacted sediment between
sandstone/shale depoaxes in which greater compaction has occurred. "Most regional
contemporaneous fault systems in the Texas coastal area consist of comparatively simple
down-to-basin faults that formed during times of shoreline regression, when periods
of fault development were relatively short" (Bruce, 1973). Cross-sectional view shows
that these faults flatten and converge at depth to planes related to fluid pressure and
form the seaward flanks of underlying shale masses. Faults, which formed during
regressive phases of deposition, "developed primarily as a result of differential com-
paction of adjacent sedimentary masses." They die out at depth near the depoaxes of the
sandstone/shale sections (Bruce, 1973).
Gravitational faults developed where the subsidence exceeded the rate of deposition
and a basinward seafloor inclination was established in the area of deposition. Post-
depositional faults are common on the landward flanks of deeply buried linear shale
masses. Many of these faults dip seaward and intersect the underlying low-density shale
at relatively steep angles (Bruce, 1973).
Bruce (1973) supported the concept of regional contemporaneous fault development
through sedimentary processes where thick masses of shale are present and where
deep-seated tectonic effects are minimal.
A schematic dip section through the Rio Grande Embayment, illustrating strata
thickening across growth faults (Murray, 1961), is shown in Fig. 8-5. Three basic types
of such regional contemporaneous faults (Bruce, 1973) are presented in Fig. 8-6, with
differentiation based on the rates of deposition of sandy sediments upon unconsolidated
clay surfaces.
According to Bruce (1973), (1) two of these types are considered to be associated
with seafloors which were relatively flat at the time of deposition, and (2) the third
type appears to be formed in areas of slope environments where seafloor subsidence
exceeded the rate of deposition.
The first example (Fig. 8-6a) represents faults formed during a regressive sequence of
deposition (progradation locally), when the amount of sediment available for deposition
was greater than the space available for accumulation. Under these conditions each
successive depoaxis was formed seaward from that of the adjacent underlying unit.
Antiregional dip, developed adjacent to the downthrown sides of these faults, varies in
relation to the amount of sediment deposited. In areas where 'still-stand depositional
conditions' prevailed, the rate of faulting was sufficient to accommodate all incoming
sediments (Fig. 8-6b). In these areas, a strong antiregional dip developed that increased
with depth and time. "Contemporaneous faults, formed during still-stand and regressive
phases of deposition, are common in southern Texas and are considered to have
developed primarily through differential compaction associated with relatively flat
seafloors" (Bruce, 1973).
Faults formed during transgressive phases of deposition are less common than the
other two types in southern Texas (Bruce, 1973). They occur when subsidence exceeds
the rate of deposition (Fig. 8-6c). The seafloor is considered to have been inclined
basinward at an angle related to the rate of subsidence. "The primary cause of seafloor