Abstract: The Double Nature of the Updip Hackberry Unconformity of Southwest Louisiana (Middle Frio)

Joel K. Jordan

Two episodes of erosion created the updip Hackberry Unconformity in southwest Louisiana north of the row of salt domes in T9S.

The first episode was one of widespread planar erosion. It occurred after the deposition of the middle Marginulina texana, when the folded and faulted pre-Hackberry strata were subaerially eroded to a relatively flat surface. The pre-Hackberry and subsequent structures are mostly unrelated: exceptions being the salt domes, the easternmost area of Jefferson Davis Parish, and a few faults.

The second episode of erosion was one of channelization by a headward erosion process of repeated slumping. It occurred, for the most part after the first episode, as the planar unconformity was tilted southward and transgressed. The slumped pre-Hackberry sediments disaggregated and moved down the steep-walled channels as debris flows. As a function of differing settling rates they were segregated and deposited as sand and shale beds in the basal portion of the same channels whose headward extension had caused their dislocation. As the channels ate their way further updip they encountered progressively more arenaceous beds, thus those channels which reach further updip have sandier basal Hackberry deposits than do those of less northward extent. In many places the channels eroded beyond the updip limit of the planar unconformity.

In mapping the updip Hackberry unconformity it is essential to regard it as two different unconformities, mapping the planar surface first and then superimposing the channels.

AAPG Search and Discovery Article #90955©1995 GCAGS 45th Annual Meeting and Gulf Section SEPM, Baton Rouge, Louisiana