Calcite-filled veins of the Austin Chalk Formation: Using a combination of geochemistry and structural geology to constrain the post-depositional history.

摘要

Twenty nine calcite-filled veins and coexisting whole rocks from the three stratigraphic members of the Upper Cretaceous Austin Chalk in Dallas and Ellis Counties were collected and analyzed for their stable isotope ratios in order to constrain the conditions of vein formation associated with normal faulting. The purpose of this thesis is to determine whether the 18O-depleted veins of the Austin Chalk are the result of the infiltration of an extraformational fluid, or if they are due to burial and increasing temperature at time of precipitation. Carbon and oxygen isotope ratios were determined for chalk and calcite-filled veins. The chalk samples range from 1.0 / < d13C < 2.3 / (mean, 1.7 +/- 0.4) and vein samples have a range of 0.1 / < d13C < 2.3 / (mean, 1.7 +/- 04). The range of d18O values for the chalk samples is -3.2 / < d18O < -5.5 / (mean -4.0 +/- 0.6). The d18O values of the calcite-filled veins fall into two distinct clusters of -5.4 / < d18O < -7.3 / (mean -6.5 +/- 0.4), present in all three members, and -7.5 / to -9.5 / (-8.5 +/- 0.5), present mainly in marl-rich middle member. Detailed drilling of five veins illustrates that the d18O values within a single cm-scale vein vary at the same level of heterogeneity (tenths of a per mil) as at the member level. The veins in the Austin Chalk are generally comprised of multiple generations of vein growth in open space that is localized in asperities along the faults. The largest accumulation of vein material is observed where faults curve or where there is a kink in the fault. Striations on the outer surfaces of the veins indicate the faults have experienced multiple episodes of faulting, even though the general displacements are only of meter scale. A dominant strike trend of N-NE is present in all three stratigraphic members. Dip degrees and dip directions vary, with the majority of the veins dipping at angles higher than 50°. The center of the distribution is between 60° and 70°, which is consistent with normal faulting. An integration of the geochemical, petrographic, and structural data for the calcite-filled veins suggests that they precipitated from the pore waters of the Austin Chalk in an essentially closed system. The isotope data suggests the veins formed at temperatures of 39°C to 52°C, which corresponds to a burial depth of at least 0.7 km. Structural data, as well as porosity and strength measurements from the literature, are in agreement with this interpretation. Based on a synthesis of the data, the deformation and formation of calcite-filled veins was driven by burial and compaction of the unit.