--> ABSTRACT: Linked Extensional-Compressional Structures in Deltaic Settings, by Robert G. Hickman and Charles J. Stuart; #90906(2001)

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Robert G. Hickman1, Charles J. Stuart2

(1) Structural Solutions, Sugar Land, TX
(2) Consultant, Park City, UT

ABSTRACT: Linked Extensional-Compressional Structures in Deltaic Settings

Extensional faults linked by decollements to downslope toe thrusts and folds are important features of deltas. Basin geometry and depositional cycles strongly influence the geometry of these systems and associated plays. The paleo-Amu Darya (pAD) and Krishna-Godavari (K-G) deltas illustrate contrasting styles.

The pAD delta prograded 85 km into the S. Caspian across a flat seafloor. Early basinward-dipping normal faults soled into a thin unit of prodelta shales. With greater displacement, deltaic sediments of the hanging wall "bottomed out" against older sediments and extension shifted to counter-regional faults. This cycle was repeated several times, producing a series of grabens above a shale weld. Turtles of deltaic sediment were left in the intervening areas. Basinward, extension was accommodated by diapiric folds cored by prodelta muds.

In contrast, the K-G deltas developed along a steep margin and the deltas prograded much less. Thick zones of over-pressured prodelta shales are present. Basinward-dipping extensional faults are linked to major toe thrusts. These features were probably active during high stands when deltaic deposition loaded the shelf. In some cases extensional faults were reactivated during new deltaic cycles, in others, new extensional faults developed basinward and dismembered older toe thrusts, in still others new extensional faults soled into condensed sections above older toe thrusts.

In the pAD system, graben faults and turtles form potential traps; outboard folds are likely shale prone. In the K-G, traps include fault rollovers, tilted fault blocks and toe-thrust folds. Ridges produced by toe thrusts may have trapped younger deepwater sands.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado