AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
The Pillow Fold Belt: A Key Subsalt Structural Province in the Northern Gulf of Mexico
(1) University of Texas, Bureau of Economic Geology, Austin, TX.
Conventional fold belts have easily recognizable folds and thrusts, but pillow fold belts are harder to recognize because shortening is subtle and superposed. Pillow fold belts contain (1) preshortening halokinetic pillows and (2) synshortening features such as thrusts, contractional kink bands, box folds, and local, strong onlap and truncation.
The Atwater fold belt is generally thought to die out landward of the frontal high-relief folds. We propose instead that landward from the Sigsbee Escarpment, the frontal anticlines pass into a pillow fold belt that continues to the present Louisiana coastline as an ultradeep shelf play, like the Davy Jones discovery. In the pillow fold belt, pillows of irregular size, shape, spacing, and orientation grew first by halokinesis then by shortening. Polygonal salt ridges link the pillows and enclose oval minibasins. Normal faults of variable strike formed on the stretched crests of pillows. Subordinate thrusts formed during shortening. In the pillow fold belt of the ultradeep Louisiana shelf, mid-Miocene subcanopy shortening is juxtaposed with late Miocene supracanopy extension. This anomalous relationship can be explained by two hypotheses. The first hypothesis is that interior uplift and coastal tilting intensified in the Miocene, building a long basinward slope far inland from the Miocene shelf break and including the interior salt basins. This tilting drove a lower linked system detached on autochthonous salt, causing shortening over a wide area independently of the Miocene shelf break. An upper linked system detached on Miocene salt canopies which extended and advanced seaward on the upper slope. Lower and upper linked systems explain why extension above the canopy was almost coeval with shortening below the canopy. Neogene uplift of the interior is signaled by faults and onlaps on the tilted margin, volume balancing of erosion and sediments, fission-track analysis, neotectonic geomorphology, and earthquakes. As a second hypothesis, reactivated shear along crustal transfer faults striking northwest could cause shortening. But crustal shear has such a wide range of structural effects that it is difficult to prove or disprove.
A pillow fold belt and two independent detachments explain how halokinesis, shortening, and extension are juxtaposed in time and space in the subsalt play.