EDWARDS, MARC B. , Marc B. Edwards Consulting Geologists Inc., Houston, Texas
In the vicinity of the shelf margin, large scale rotated blocks, form through two endpoint processes: 1) gradual creep of the continental slope that forms the familiar growth fault systems, and 2) abrupt collapse of the shelf margin that forms large-scale erosion-detachment surfaces overlain by rotated blocks.
The first process involves sedimentation simultaneous with structural creep and block rotation, resulting in fanning of stratal dip in basinward-thinning wedges. Density instability above a deformable substrate can raise the basinward part of the wedge causing erosional truncation. Since the dynamic sedimentary-tectonic system is maintained by an active sediment supply, depositional environments are generally shallow water.
The second process produces blocks that generally do not show fanning of dip because block rotation occurs after deposition. Collapse of the shelf margin results in a sudden change from shallow water to deep water conditions. Blocks are retransported into deep water where they discordantly overlie older strata, and are unconformably overlain by deep water deposits, including hemipelagic shales and gravity flow sandstones.
Subsequent progradation into the collapse-formed depression produces inclined strata that resemble the lowstand prograding wedge of sequence stratigraphy. However, this architecture is primarily due to the collapse-formed unconformity and subsequent flexural rebound of the shelf margin, rather than eustatic forcing.
Examples of rotated blocks and their relationship to the distribution and characteristics of sandstone reservoirs will be provided for the Wilcox, Frio, Yegua and Lower Miocene producing trends of Texas and Louisiana.
AAPG Search and Discovery Article #90908©2000 GCAGS, Houston, Texas