Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
AAPG Annual Convention, Calgary, Alberta, Canada, June 19–22, 2005
A Compressional Origin for Minibasins Near the
The conventional explanation for minibasin subsidence
is that it is driven by gravity—that minibasins exist because their
fill is dense enough to sink into the underlying evaporites, expelling
salt into the adjacent salt highs. This explanation is valid if the average
density of the sediments is greater than the density of the salt, but
it cannot account for subsidence of thin, less dense clastic sequences
into salt. Seismic data show that many minibasins started sinking into
salt when their siliciclastic fill was much thinner than the 1.5–2
km thickness necessary for compaction to invert the density contrast.
For such minibasins, some mechanism other than gravity must be involved.
We investigated mechanisms of minibasin subsidence using a 3,600-km2 pre-stack
depth-migrated 3D seismic dataset near the Sigsbee Scarp, northern Gulf
of Mexico. This dataset covers 27 minibasins of varying size and thickness.
These data indicate that minibasin initiation was synchronous with shortening,
as indicated by the presence of thrust faults in the deeper parts of many
minibasins. A compressional origin of minibasins is also consistent with
finite-element models showing that laterally shortened minibasins will
subside even if their fill is less dense than the salt. The specific cause
of shortening that led to minibasin formation is currently unknown. The
orientation of thrust structures is highly variable. Their pattern suggests
that shortening was partitioned by flow boundaries defined at shallow
levels within and above the salt sheet. If so, suprasalt processes may
have been an important control.