Juan Carlos Laya

Clumped isotope thermometry in deeply buried sedimentary carbonates: The effects of bond reordering and recrystallization

We utilize carbonate clumped isotope thermometry to explore the diagenetic and thermal histories of exhumed brachiopods, crinoids, cements, and host rock in the Permian Palmarito Formation, Venezuela, and the Carboniferous Bird Spring Formation, Nevada, USA. Carbonate components in the Palmarito Formation, buried to ∼4 km depth, yield statistically indistinguishable clumped isotope temperatures [ T (Δ47)] ranging from 86 to 122 °C. Clumped isotope temperatures of components in the more deeply buried Bird Spring Formation (>5 km) range from ∼100 to 165 °C and differ by component type, with brachiopods and pore-filling cements yielding the highest T (Δ47) (mean = 153 and 141 °C, respectively) and crinoids and host rock yielding significantly cooler T (Δ47) (mean = 103 and 114 °C). New high-resolution thermal histories are coupled with kinetic models to predict the extent of solid-state C-O bond reordering during burial and exhumation for both sites. Application of these models, termed “THRMs” (thermal history reordering models), suggests that brachiopods in the Palmarito Formation experienced partial bond reordering without complete equilibration of clumped isotopes at maximum burial temperature. In contrast, clumped isotope bonds of brachiopods from the Bird Spring Formation completely equilibrated at maximum burial temperature, and now reflect blocking temperatures achieved during cooling. The 40–50-°C-cooler clumped isotope temperatures measured in Bird Spring Formation crinoids and host rock can be explained by recrystallization and cementation during shallow burial combined with a greater inherent resistance to solid-state reordering than brachiopods.

Controls on diagenesis and dolomitization of peritidal facies, Early Cretaceous Lower Edwards Group, central Texas, USA

The Early Cretaceous Fort Terrett Formation of Mason County, central Texas, is a succession of subtidal to peritidal mud-dominated facies with minor intervals of bioclastic packstone–grainstone, rudist floatstone, and interbedded chert nodules. The strata conformably overlie the Hensel Formation, which was deposited unconformably on Precambrian basement. The Hensel Formation also contains a significant percentage of dolomite, precipitated within a fine-grained clayey matrix. The Hensel and Fort Terrett Formations were deposited during a transgressive episode, which provided the conditions for the extensive shallow-water Comanche carbonate platform. Siliciclastic and carbonate sediments were deposited along the coastal margin in subtidal, intertidal to supratidal areas. Previous dolomitization models have suggested that high permeability layers are required for dolomitizing brines to flow through a carbonate succession. Although, interparticle porosity in muddy tidal-flat successions can be significant, it has a limited flow capacity. However, interconnected fenestral porosity can allow sufficient fluid flow to move dolomitizing fluids more efficiently through the succession. Thus, it is hypothesized that interconnected fenestral porosity could have had a significant impact on permeability within this muddy succession and provided the pathways and conduits for Mg-rich brines. Four types of dolomite are recognized in the Fort Terrett succession. Three of these dolomite types formed largely by replacement and they occur throughout the succession. Features such as crystal size, crystal face geometry and zonation reflect the progressive development and recrystallization of the dolomite types. Only type 4 dolomite formed as a cement in void spaces during a late diagenetic stage. The direction of the dolomitizing fluid movement is difficult to determine, but it was likely downward in this case, controlled by a density-head driving-mechanism generated by dense hypersaline fluids from an evaporating lagoon.