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Fluvial Sedimentology and Architecture of the Middle Frio Formation (Oligocene), Jim Wells, Klegerg, and Nueces Counties, South Texas

KERR, DENNIS R., Bureau of Economic Geology, University of Texas at Austin, Austin, TX

The middle Frio Formation of the northeastern Gueydan fluvial system comprises fluvial and overbank deposits averaging 2000 to

3000 ft in thickness. Discrete genetic intervals are defined from subsurface studies using four cores totaling 575 ft, high-resolution resistivity borehole images, and electric logs. A genetic interval is composed of four facies: (1) Channel-fill facies (30-ft thick and 2500-ft wide) includes an overall fining-upward texture and upward diminution in sedimentary structure scale: intraclast gravel lag; large-scale trough cross-stratified, medium-grained sandstone; parallel-stratified and ripple-laminated fine-grained sandstone to sandy mudstone containing pedogenic nodules and root molds. Departure from the overall vertical trend within the channel-fill facies is common and is regarded as the result of chute-channel development processes. (2) Splay fine-grained sandstones (up to 20-ft t ick) are parallel-stratified and ripple-laminated and are organized into beds with variable vertical thickness trends. (3) Levee sandstones and mudstones are thinly bedded with low-angle surfaces and slump structures. (4) Floodplain mudstones contain structures and fabrics indicating variable wet and dry conditions (e.g., vertisol-type paleosols).

Three scales of architectural elements are readily identifiable in the subsurface. (1) Multitiered lateral accretion bars are the principal channel-filling element. (2) Levee-confined channel systems have a lower tortuosity than the inferred thalweg, and splay complexes extend up to 2 mi from channel systems. (3) Discrete genetic intervals form a continuum of stacking patterns: from laterally stacked, where sandstone bodies contact one another or are vertically separated by only a few feet of the floodplain facies, to vertically stacked, where floodplain facies substantially separate sandstone bodies. Rate of coastal-plain aggradation is believed to be the governing control on stacking patterns: lower rate leading to lateral stacking and higher rate leading to vertical stacking.


AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)