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Abstract Extensive regions of yellow and white (“bleached”) sandstones within the terrestrial Jurassic red bed deposits of the Colorado Plateau reflect widespread interaction with subsurface reduced fluids which resulted in the dissolution of iron‐oxide grain coatings. Reduced fluids such as hydrocarbons, CO2, and organic acids have been proposed as bleaching agents. In this study, we characterize an altered section of the Slick Rock member of the Jurassic Entrada Sandstone that exposes bleached sandstone with bitumen‐saturated pore spaces. We observe differences in texture, porosity, mineralogy, and geochemistry between red, pink, yellow, and gray facies. In the bleached yellow facies we observe quartz overgrowths, partially dissolved K‐feldspar, calcite cement, fine‐grained illite, TiO2‐minerals, and pyrite concretions. Clay mineral content is highest at the margins of the bleached section. Fe2O3concentrations are reduced up to 3× from the red to gray facies but enriched up to 50× in iron‐oxide concretions. Metals such as Zn, Pb, and rare‐earth elements are significantly enriched in the concretions. Supported by a batch geochemical model, we conclude the interaction of red sandstones with reduced hydrocarbon‐bearing fluids caused iron‐oxide and K‐feldspar dissolution, and precipitation of quartz, calcite, clay, and pyrite. Localized redistribution of iron into concretions can account for most of the iron removed during bleaching. Pyrite and carbonate stable isotopic data suggest the hydrocarbons were sourced from the Pennsylvanian Paradox Formation. Bitumen in pore spaces and pyrite precipitation formed a reductant trap required to produce Cu, U, and V enrichment in all altered facies by younger, oxidized saline brines.
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Rapid morphological change in the Late Triassic bivalve Monotis from Aotearoa New Zealand: Environmental or Biological Drivers?
In the South Island of Aotearoa (New Zealand), the preservation of biogenic carbonate in Late Triassic sedimentary rocks is rare to non-existent; however, differential preservation modes between common phyla are commonly observed and serve to elucidate the stratigraphic and diagenetic history of these often poorly- exposed immature sandstone units. The Taringatura Group sandstones from Southland and Otago range from sandy siltstones to silty arkosic sandstones that commonly host molluscan and brachiopod macrofossils as well as rare echinoderms, bryozoans, and foraminifera. Additionally, there is a hypothesized unconformity between the lower Oretian and Otamitan age (227.7–217.0 Ma) and the overlying Warepan age (217–208.5 Ma) deposits indicated by an abrupt change in composition, grain size, and fossil assemblage. Molluscs from the Oretian and Otamitan deposits exhibit fine-detail preservation on external and internal molds. Thin-shelled taxa, such as Halobia, exhibit some shell replacement by clay minerals, likely from the dissolution of feldspars in the surrounding rock. Conversely, larger and thicker-shelled brachiopods and bivalves such as Manticula and Hokonuia do not present as casts. When preserved, foraminifera and rare bryozoans are typically silicified. The overlying Warepan sandstone beds frequently contain fossils of the bivalve Monotis which exhibit a similar preservation style to older molluscs, though lacking clay minerals. Presently, the fossiliferous Taringatura sandstones exhibit low porosity and low permeability, as is expected from the subsequent compaction of sandstones after burial. However, the dissolution of biogenic carbonate implies a past permeability. The presence of clay minerals in Oretian and Otamitan fossils may indicate a period of subaerial exposure and infiltration of meteoric water prior to the deposition of Warepan units. Notably, clay replacement occurs more frequently in the thinnest fossils. Original carbonate material may have persisted for longer in the more robust taxa, allowing them to resist most deformation from compaction prior to the final loss of carbonate. Differential diagenesis of biogenic carbonates supports the existence of a significant unconformity between Otamitan and Warepan units in the Taringatura sandstones.
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- Award ID(s):
- 1654088
- PAR ID:
- 10538664
- Publisher / Repository:
- Geological Society of America Abstracts with Programs
- Date Published:
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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