Chalk Canyons

Tallahatta - Age Canyons Backfilled With Chalk

This discussion extracted

from the presentation awarded

Best Paper of 2001

by the

New Orleans

Geological Society

(NOGS)

Above: Cretaceous Chalk gravel from Eocene canyon fill, Warren County, MS.

Right: Portion of mudlog illustrating 75% Cretaceous Chalk gravel in drill cuttings.

Recent investigations by Steve Walkinshaw, President of Vision Exploration, have resulted in the identification of several prominent canyons that have been eroded into the Wilcox Formation in East Louisiana and West Mississippi. In a study most recently confirmed by paleontology performed at Ball State University, Steve documents the surprising discovery that the gravel fill which comprises the bulk of the redeposited sediments in these previously undocumented Tertiary canyons is composed of Upper Cretaceous Chalk. These relatively deep (> 400') canyons appear to be Tallahatta (Eocene) age and have been observed in Montgomery, Warren, and Copiah Counties in Mississippi and East Carroll Parish, Louisiana. They are readily apparent on modern 2D and 3D seismic data as erosional features that downcut into the underlying Wilcox section of sand and shales. Seismic events within the canyons that exhibited drape over the canyon axes, also clearly visible on the seismic data, were poorly understood until it was determined that the denser Chalk gravel - deposited in the thalweg of the canyons - had caused differential compaction of the overlying Tertiary shales as they were deposited over the harder Cretaceous sediments.

The geographic source of the Chalk gravel that comprises the canyon fill is still very much unknown. While the location of the Chalk/Tertiary subcrops has been generally identified, their considerable distance from the canyons in west Mississippi have challenged geologists to rethink their assumptions regarding sea level fluctuation, sediment transport, and paleobathymetry during Tallahatta time. The linkage of these "side" or "feeder" canyons with the large, ancestral Mississippi River canyon developed in the subsurface of southwest Mississippi and south-central Louisiana is suspected but not yet proven. Certain implications for Yoakum Canyon - type traps are inferred, but to date uncertain.

Subsurface Evidence of Chalk-Filled Canyon (Warren County, Mississippi)

Portion of structural cross-section utilizing well control in Warren County, Mississippi that dramatically illustrates the erosion and back-filling of a large Tallahatta-aged canyon cut almost completely through a thick Upper Wilcox sand & shale section (note the abrupt change in the SP log character as one traverses from flank to center). Wells that penetrated the canyon fill encountered Cretaceous Chalk gravels deposited within the canyon in lateral juxtaposition with younger Wilcox sediments; studies indicate the Chalk gravels were eroded from a nearby Chalk outcrop exposed during Tallahatta time, transported downdip into the canyon, and quickly buried.

Chalk Gravel Recovered From Well That Penetrated The Canyon Fill

Illustrated above is a photograph of the actual Cretaceous Chalk gravel that was recovered from drill cuttings encountered when the canyon fill was penetrated in a well drilled through its deepest section. This Chalk gravel was redeposited in what is normally an Eocene Wilcox sand and shale section. Over 400 feet of this remarkable gravel deposit was encountered in the subject well, the drilling of which was personally supervised by Steve Walkinshaw. A portion of the mudlog for the well provides a graphic record of the Chalk gravel cuttings.

2D Seismic Line Illustrating Canyon Cut Through Selma Chalk (Louisiana)

(Note: Separate, Downdip Erosional Event)

Portion of long (15-mile) east-west oriented 2Dseismic line illustrating 12 mile-wide canyon cut through most of the Wilcox and almost 1,000 feet of the underlying Cretaceous Chalk (Selma-Austin age); in this case, well control that has penetrated this erosional event indicates it is back-filled with Eocene shale and minor lenses of sand. This seismic line is from the south-central Louisiana area, downdip of the chalk-filled canyon described above; tracking this (separate) canyon further south reveals the erosion of not only the entirety of the Chalk but also the underlying Eagleford and Marine Shales of the Upper and Middle Tuscaloosa (or equivalent) Formation. At its deepest & widest points, this canyon removes over 3,000 feet of older sediments and spans 13 miles in width.