--> Abstract: Evaluating Evaporite Karst Expressions in Outcrop Using Terrestrial LiDAR Within the Mississippian Madison Formation, Fremont Canyon, Wyoming, by Joseph El-Azzi and Charles Kerans; #90124 (2011)

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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Evaluating Evaporite Karst Expressions in Outcrop Using Terrestrial LiDAR Within the Mississippian Madison Formation, Fremont Canyon, Wyoming

Joseph El-Azzi1; Charles Kerans2

(1) Bureau of Economic Geology, University of Texas at Austin, Austin, TX.

(2) Dept. of Geological Sciences, University of Texas at Austin, Austin, TX.

One of the largest remaining carbonate hydrocarbon resources in the world is within the bitumen-rich Devonian Grosmont Formation. Core and log data highlight the importance of stratiform, evaporite karst as a reservoir characterization issue within this play. Internal complexities related to evaporite karst are numerous as demonstrated by outcrops of the Mississippian Madison Formation in Fremont Canyon, Wyoming.

A combined approach was used to map and quantify outcrop karst features, including: (1) terrestrial LiDAR to survey the outcrop at 0.3 cm resolution; (2) photomosaics, and (3) measured stratigraphic sections through the upper Madison Fm. and (4) stratigraphic contact mapping that was geospatially referenced using high-resolution GPS. Thin sections and XRD analyses were conducted on samples collected along a vertical transect containing both post-Madison karst fill and Madison Limestone host rock to determine fabric and mineralogical composition.

Two fundamentally different karst expressions were observed within the upper Madison sequences and were classified as intra-Madison and supra-Madison karst. Intrastratal evaporite-solution karst has a 5 m thick cave-fill unit with laminated sediment and sub-rounded (transported) cm-scale Madison breccia clasts. Sediment and clasts are predominantly dolomitic with minor amounts of matrix quartz sand and clay. Above the intrastratal cave fill there is a consistent progression from a coarsening upward chaotic breccia to a fractured, mosaic breccia interpreted as ceiling collapse below disturbed paleocave roof. This is followed by a succession of intact host-rock limestone. The entire succession has an average thickness of ~25 m. Coincident to the intrastratal karst, a suprastratal, solution doline karsted surface developed along the Mississippian-Pennsylvanian unconformity. This surface is highlighted by the deposition of the Darwin Sandstone into the dolines which have an average vertical relief of ~10 m with maximum peak width spaced between 15-40 m. The resultant characterization of observed karst-related irregularities and associated karst facies provide key statistical information used to create a three-dimensional model of the karst system.