Abstract

Faults along which movement was contemporaneous with sediment deposition are common in the post-Eocene beds of the Gulf Coast. These faults have been called "depositional faults," "progressive faults," and other names, but the term "contemporaneous fault" has priority in geological literature and should be used to describe faults of this type. Increase in the thickness of beds on the downthrown side of a fault when compared with the same beds on the upthrown side is generally considered to be evidence for a contemporaneous fault. In general, the younger the sediments, the more numerous are contemporaneous faults. Faults of this type in the Gulf Coast are largely confined to a belt paralleling the coast line and located downdip from the Vicksburg (Oligocene) flexure. Since movement occurs contemporaneously with deposition, contemporaneous faults control sedimentation in areas affected by the movement. The attitude of the depositional surface of the downthrown fault block largely determines the distribution of sediments over the fault block. A flexure is a zone downdip from which the rate of dip and thickening of sedimentary beds is accentuated. The writers believe that flexures result from deposition of sediments across the shelf break. Flexures affect most Cenozoic beds in the Gulf coast, and most flexures are marked by contemporaneous faults of regional extent which tend to accentuate the effect of a flexure on sedimentation. Sedimentation rate in the Gulf Coast has been steadily increasing since Cretaceous. Maximum sedimentation rate has risen from 2 cm. per century for Eocene sediments to 8.1 cm. per century for Miocene sediments and 12.2 cm. per century for Pleistocene and Recent sediments. The prevalence of contemporaneous faults in post-Eocene beds is apparently related to this increase in rate of sedimentation and subsidence. Regional contemporaneous faults were formed along zones of flexure as a result of slumping, but contemporaneous faults associated with local structures are the result of normal structure-forming tectonic forces at work in the Gulf Coast during periods of rapid sedimentation.

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