Salt-Ridge Hypothesis on Origin of Texas Gulf Coast Type of Faulting

Abstract

The salt-ridge hypothesis explains the origin of a type of normal faulting found abundantly in the salt-dome province of thick Tertiary sediments bordering the Gulf Coast of Texas. This hypothesis states that deep-seated vertical intrusions of salt in the form of long narrow ridges push up the gulfward dipping beds along their strike to form deep-seated anticlines. A pattern of normal faults with dips of approximately 45° has to develop on every anticline as an adjustment for the vertical forces involved. Depressions or edge synclines may parallel the ridges on one or both sides as a result of salt removal. Two main hypothesis of origin which have previously received the greatest consideration among geologists are: (1) subsidence of the basement rock toward the Gulf, and (2) slipping of sediments downdip toward the Gulf on a bedding plane. Neither the subsiding-basement nor the slipping-plane hypothesis is found to be adequately in agreement with present evidence. The salt-ridge hypothesis is the only one of the three hypotheses considered that satisfactorily explains all of the following nine Gulf Coast fault characteristics that are shown on seismograph sections: (1) approximately 45° angles of fault-plane dip, (2) both up-to-the-coast and down-to-the-coast major movement along the same fault trend, (3) adjustment faults forming narrow grabens, (4) dipping down of beds into the downthrown side of faults, (5) dipping up of beds under the upthrown side, (6) great increase of throw with depth, (7) abrupt dying-out of faults laterally, (8) localized throw across fault with no relative displacement of beds a mile or more away, and (9) a decrease with depth in the amount of formational dip down into the downthrown side of the fault. The presence of salt ridges would require that new concepts be used for locating deep oil fields. Favorable structures would be far more numerous than are now envisioned, and the location of test wells with respect to the basic salt anticline would be relatively simple.