Gary G. Gray

Three-Dimensional Implicit Stratigraphic Model Building From Remote Sensing Data on Tetrahedral Meshes: Theory and Application to a Regional Model of La Popa Basin, NE Mexico

Remote sensing data provide significant information to constrain the geometry of geological structures at depth. However, the use of intraformational geomorphologic features such as flatirons and incised valleys often calls for tedious user interaction during 3-D model building. We propose a new method to generate 3-D models of stratigraphic formations, based primarily on remote sensing images and digital elevation models. This method is based on interpretations of the main relief markers and interpolation of a stratigraphic property on a tetahedral mesh covering the domain of study. The tetrahedral mesh provides a convenient way to integrate available data during the interpolation while accounting for discontinuities such as faults. Interpretive expert input may be provided through constrained interactive editing on arbitrary cross-sections, and additional surface or subsurface data may also be integrated in the modeling. We demonstrate this global workflow on a structurally complex basin in the Sierra Madre Oriental, Northeastern Mexico.

Detrital zircon geochronology from the Bighorn Basin, Wyoming, USA: Implications for tectonostratigraphic evolution and paleogeography

Tectonostratigraphic assemblages record phases of basin history during which the fundamental controls of tectonic setting, subsidence style, and basin geometry are relatively similar. Because these fundamental controls, in combination with climate and eustasy, influence paleogeography and sediment-dispersal patterns, they should also yield similar patterns, or facies, of detrital zircon age spectra. Such age-distribution patterns should be documented on the craton in order to make meaningful comparisons to sedimentary rocks from suspect terranes along continental margins. The Rocky Mountains of western North America provide excellent outcrops of sedimentary rocks that record >500 m.y. of tectonostratigraphic evolution. One such Phanerozoic section is exposed along the margins of the Bighorn Basin in northwest Wyoming, from which we report over 4000 U/Th/Pb detrital zircon ages from 48 samples that span a stratigraphic interval from the Middle Cambrian Flathead Sandstone through the Eocene Willwood Formation. These data provide one of the most complete records of detrital zircon age patterns from this part of cratonic North America. The stratigraphic record of the Bighorn Basin is subdivided into four tectonostratigraphic assemblages (TSA1–TSA4). These assemblages record an initial passive margin, followed by a transition to a convergent margin, followed by a marine-dominated retroarc foreland basin, followed by a retroarc foreland segmented by local basement uplifts. This tectonostratigraphic architecture is expressed as four, first-order patterns within the detrital zircon age distributions. TSA1 represents a Paleozoic–Triassic proximal continental margin assemblage dominated by Proterozoic zircons with abundant grains in the 1600–1950 Ma range, a Grenville population at ca. 1100 Ma, and a Phanerozoic population at ca. 420 Ma. TSA2 is a transitional assemblage associated with the Jurassic–Early Cretaceous organization of a west-facing convergent margin and Cordilleran orogen. The TSA2 detrital zircon age distribution is characterized by the appearance of Mesozoic grains, age peaks at ca. 420 and 600 Ma, and a dominant population of Grenville (1.0–1.1 Ga) grains with a suite of Proterozoic grains diminishing in abundance as age increases to 1.9 Ga. TSA3 sedimentary rocks were deposited in the Cretaceous Interior Seaway in a retroarc foreland basin and are dominated by zircons for which ages are close to the depositional age of the strata, reflecting input from the active Idaho Batholith and Sierran segments of the Cordilleran magmatic arc. The older zircon fractions from TSA3 sedimentary rocks are characterized by a dominant detrital zircon age peak at 1.7–1.8 Ga, which probably reflects reworking of Belt Supergroup metasedimentary rocks from the northwest into the Cretaceous foreland, based on regional paleogeographic patterns. TSA4 reflects the phase of basin fill associated with Paleogene structural segmentation of the retroarc foreland during the Laramide orogeny. Detrital zircon age spectra from this assemblage record erosion and redeposition of all previous sedimentary rocks from surrounding basement uplifts. Patterns of detrital zircon ages reflect fundamental changes in paleogeography and sediment dispersal at the 10–100 m.y. time scale and are clearly related to major tectonic events or phases. Detrital zircon ages also provide evidence for linkages between convergent margin processes such as arc magmatism and sedimentation in the retroarc foreland. During these times of strong arc-retroarc linkage, detrital zircon geochronology provides a potentially useful tool for high-resolution chronostratigraphy.

A palinspastic Mesozoic plate reconstruction of New Zealand

Abstract The restoration of New Zealand into a plate reconstruction of Gondwanaland requires a complex process of fitting the constituent plates and removing large amounts of post-breakup deformation. We have modified earlier plate reconstructions of New Zealand to provide the tightest fit possible of all of the relevant plates prior to palinspastically restoring the New Zealand plates to a pre-breakup configuration. We have used the trend of Permian ophiolite fragments and their offshore magnetic expressions as our datum for both the reconstruction and restoration, and have tested two models for removing the “oroclinal bending” of the ophiolite belt: constant-length restoration (literally “unbending”), and shearing parallel to the present-day plate boundary. Our analysis shows that restoration using the shearing model provides the most reasonable plate outline for use in the plate reconstructions. Approximately 1000 km of translation along the Alpine fault system is removed by this combination of plate reconstruction and palinspastic restoration. Paleogeographic constraints on both the reconstruction and restoration are greatest in the Triassic, becoming non-diagnostic by the Late Cretaceous. Further improvements to this palinspastic restoration can be made by removing the ~100 km of compressional deformation along the Alpine fault system, and by slight changes in the major plate reconstruction.

Deciphering the diagenetic history of the El Abra Formation of eastern Mexico using reordered clumped isotope temperatures and U-Pb dating

Carbonates form ubiquitously throughout the history of deposition, burial, and uplift of basins. As such, they potentially record the environmental conditions at the time of formation. Carbonate clumped isotopes provide the temperature of precipitation but can be internally reordered if the host rock is exposed to elevated temperatures over geologic time scales. Here, we exploited this kinetic behavior by analyzing multiple generations of cements that capture the range of environments experienced by the El Abra Formation from eastern Mexico. From this, we developed a quantitative diagenetic history for these different phases of cementation. We observed a 70 °C range in clumped isotope temperatures from 64 °C to 134 °C for these cements, which is not compatible with their inferred precipitation environments. This suggests that bond reordering occurred during burial but did not fully reorder all cements to a common apparent temperature. We reconstructed original cement growth temperatures and the isotopic signature of the parent fluids to show that precipitation from a marine pore fluid began at 125 Ma, contemporaneous with deposition, and continued throughout burial to temperatures of at least 138 °C at 42 Ma. We show that precipitation of equant cements, which occluded 90% of the pore space, was coincident with Laramide-related burial to depths greater than 3800 m. A U-Pb age of diagenetic calcite of 77.1 ± 3.6 Ma provides independent support for our estimates of the absolute timing of precipitation of two distinct phases of the paragenesis. This is the first demonstration of the utility of integrating U-Pb age dating with reordered clumped isotope temperatures to provide quantitative constraints on the time-temperature history of cementation. Such information may ultimately lead to advances in our understanding of the formational environments and geological processes that drive diagenesis in carbonates for temperatures below the clumped isotope “blocking temperature.”