STABLE ISOTOPES AND FAULT ORIENTATIONS ASSOCIATED WITH THE CONVERSION OF OOZE TO CHALK, AUSTIN CHALK, DALLAS COUNTY, TEXAS: POSSIBLE INVARIANCE OF SCALE, MUD CRACKS TO POLYGONAL FAULTS

The Cretaceous Austin Chalk exhibits large numbers of fractures and normal faults whose orientations have been attributed to regional stresses (e.g. the Balcones fault trend) or, by analogy with the North Sea chalks, to polygonal faulting resulting from contraction. Fault asperity, striated calcite veins and host-rock chalk samples have δ¹⁸O values of -9.5 < δ¹⁸O <-5.4‰ and -4 < δ¹⁸O <-3.5‰, respectively. Assuming the seafloor temperature is ~21℃ from Inoceramid -2.6 < δ¹⁸O <-2.3, the trapped Cretaceous seawater fluid (~ -1.2 ‰) suggests precipitation between 30 and 65 ℃ within the first kilometer of burial during the transformation of calcareous ooze into chalk. In Dallas County, rose diagrams of fault orientations suggest three dominant directions roughly N-S, NE-SW and E-W suggesting a polygonal minor fault structure. Using geomorphologic data (topographic and DEM data) on stream orientations suggest that the headwater (1^st or 2^nd order) streams best reflect the underlying fault patterns in the chalk. The higher order streams tend to reflect the strike and/or down dip direction of the chalk. To simulate the formation of polygonal faults, desiccation of ooze-like muds to produce mud cracks generates polygonal patterns with cracks having orthogonal, y, non-orthogonal, and crossing intersections. Preliminary results suggest a compositional dependence for the distribution of intersection types and a relationship between thickness and polygonal area.