Evidence Of Meteoric Diagenesis Research Articles

Insights into the Devonian–Carboniferous transition and Hangenberg Event from δ13Ccarb and 87Sr/86Sr chemostratigraphy of shallow platform carbonate strata of northwestern Colorado

ABSTRACTPaired chemostratigraphic and biostratigraphic data suggest that the Devonian–Carboniferous boundary and the Hangenberg extinction event are recorded in the Coffee Pot Member of the Dyer Formation of the White River uplift region of northwestern Colorado. The Hangenberg isotopic excursion interval occurs in biostratigraphically depauperate shallow platform micritic dolostone and limestone representing the aculeatus–?ultimus–kockeli? Zone. The Hangenberg interval strata have δ13Ccarb values up to 7.69‰, and locally contain ooids, stromatolites, and other microbial structures. In three sections, there is a short-lived > 4‰ “pre-Hangenberg” positive excursion that is associated with the presence of detrital dolomite siltstone. The main Hangenberg isotopic signature exists in overlying strata, and is replicated in four different sections of the Dyer Formation across an area ~300 km2. In each section, the excursion interval is preceded by, and in one location is interrupted by, evidence of meteoric diagenesis and/or emergence characterized by macroscopic dissolution features and pronounced negative δ13Ccarb values (to ca. -6‰). Conodont 87Sr/86Sr ratios through the Dyer Formation dip to nearly 0.708 during the onset of the Hangenberg Event, consistent with the brachiopod based 87Sr/86Sr nadir from the same interval in Missouri, France, and Germany. The least-radiogenic trendline through the Dyer 87Sr/86Sr record matches the global minimum 87Sr/86Sr trendline through the Famennian–Tournaisian transition and infills several gaps in the global record.

Late Mississippian limestone sedimentary environment in southern Pembrokeshire (Bullslaughter Bay, Wales): evidence of meteoric diagenesis and hypersaline features

AbstractBullslaughter Bay in southern Pembrokeshire, UK, exposes sections of Upper Mississippian limestone strata. In many places, the rock suffered an isovolumetric alteration during a period of sea-level oscillations. We used multiple approaches to study the weathered rocks, combining sedimentological, petrographic and isotopic compositions (δ18O and δ13C values). Two main microfacies are recognized: (i) packstones/grainstones, characteristic of an open marine shallow subtidal/intertidal environment, with a high degree of agitation, slightly elevated salinity and temporary subaerial exposure; and (ii) mudstones/wackestones in a lagoonal setting and intertidal or supratidal environments, with a pedogenetic influence. In both cases, a complex diagenetic story, which started early in a meteoric environment, induced a strong alteration producing loose sediments in place of the parent rock. Calcretization, at or near the sediment surface in the vadose zone, was one of the most widespread diagenetic modes. It could be associated with beachrocks. Carbon and oxygen stable isotope analyses from more or less weathered limestones support the petrographic data: they show non-marine values with δ13C ranges of from −2.13 ‰ to 1.75 ‰ and δ18O from −6.05 ‰ to −4.66 ‰. These values are systematically lower than those of the middle Carboniferous seawater. Some periods of low sea level and subaerial exposure allowed gypsum to form. Neoformation of euhedral quartz by probable replacement after sulfate, and halite pseudomorphs after gypsum in a hypersaline environment are documented for the first time in southern Pembrokeshire. The studied weathered limestones present a complex diagenetic evolution related to sea-level oscillations in a range of hot and contrasting seasonal climates.

Diagenetic Origins of the Calcite Microcrystals That Host Microporosity In Limestone Reservoirs

Abstract Micron-size pores are common in limestones, and they host significant hydrocarbon reserves in conventional and unconventional plays worldwide. Previous studies have shown that the vast majority of micropores in limestones occur in the interstices between micron-size calcite crystals that are present in nearly all depositional environments and burial depths throughout the Phanerozoic. In an effort to elucidate the genetic history of these calcite microcrystals, a geochemical study was undertaken to investigate a global dataset collected from seven new subsurface microporous reservoirs as well as data from 21 previous studies. Bulk elemental analyses indicate that the vast majority of microcrystals are composed of low-Mg calcite (average 14 mmol/mol, range 1.6–90, Mg/Ca). Microcrystal Sr/Mg ratios are consistent with abiotic calcite cements, though overall Mg/Ca and Sr/Ca are lower than modern abiotic calcite cements reported in the literature. Higher Sr/Ca observed in depositional chalks implies a biotic, though probably not aragonitic, precursor. Most calcite microcrystals have oxygen isotope compositions that are 0 to 4‰ more negative, and carbon isotope compositions just slightly more negative (< 1‰), than contemporaneous abiotic marine calcite. Most stable oxygen and carbon isotopic data plot either along a burial trend anchored at its most positive end in the age-equivalent marine-calcite composition or parallel to that trend with an offset in δ13C. Assuming that δ13C offsets are due to marine carbon isotopic excursions, the isotopic data are consistent with shallow-burial diagenesis as the dominant diagenetic pathway for most calcite microcrystals. For samples with information on porosity and depth, more negative δ18O (ca. 1–4‰) is associated with increased depth and reduced porosity, implying that progressive overgrowth of calcite microcrystals is modifying bulk chemistry and rock properties of the reservoir during burial diagenesis. A meteoric calcite line was evident only in two datasets. This demonstrates that evidence of meteoric diagenesis can be preserved in the chemistry of calcite microcrystals, but that its occurrence is rare in comparison with evidence for diagenesis during shallow-burial. Geochemical data from calcite microcrystals are bulk samples, i.e., measurements averaged over many, many calcite microcrystals which themselves may have zoned chemistries. As such, microcrystal data require additional interpretation compared to samples from calcite macrocrystals where superposition puts samples into clear chronological order and produces diagenetic trends on, for example, δ18O–δ13C plots. Bulk samples of calcite microcrystals should show mixing between phases on diagenetic trends. With this model in mind, the assembled database suggests that most samples are mixes of primary marine and burial calcite.

Drowning history of a Miocene carbonate platform (Zhujiang Formation, South China Sea)

The identification and interpretation of drowning events in the geologic record can aid significantly to the reconstruction of the depositional, tectonic and eustatic history of a study area and often improve reservoir and seal prediction in carbonate rocks. The differentiation between drowned platforms showing a record of continuous deepening and those with a record of exposure followed by rapid deepening remains, however, problematic. The Zhujiang carbonate platform (Liuhua 11-1 field, South China Sea) study shown here provides an example of an integrated approach combining high-resolution geochemistry, microfacies analyses and foraminiferal biostratigraphy in order to improve the reconstruction of environmental conditions prior, during and after platform demise and drowning. The Zhujiang carbonate platform displays the following vertical succession of four facies types i) skeletal grain facies with a miogypsinid/lepidocyclinid-dominated fauna deposited in a moderately deep (< 50 m), oligotrophic back-reef setting; ii) in situ corals in patch-reef facies in an oligotrophic lagoon (< 10 m); iii) rhodoid facies with in situ red algal crusts, dominated by Heterostegina sp. and spiroclypeids, possibly capped by a subaerial exposure surface. Well-rounded rhodoids representing a mesotrophic lagoon dominate the upper portions of the rhodoid facies; iv) pelagic marine shales of the Hanjiang Formation burying the carbonate platform after drowning. This facies succession, in combination with geochemical evidence suggests a deepening-upward trend. This trend might have been interrupted by transient subaerial exposure but no evidence for meteoric diagenesis was found at the drowning unconformity topping the carbonate platform. Instead, microfacies analyses suggest that platform demise may be related to progressive changes in environmental conditions, including increasing nutrient-levels and/or decreasing temperature up-core towards the drowning unconformity. These findings are of significance for those concerned with Miocene carbonate factories and, more specifically, the demise of carbonate platforms in general.

Porosity Formation in Deep-Burial Environment: Overview, with Examples, from Permian Basin: ABSTRACT

Porosity formation accompanying deep burial is ubiquitous and widespread in the Permian basin, particularly but not exclusively in offshore platform and resedimented basinal carbonates of Pennsylvanian and Permian age. Hydrocarbon reservoirs in such platform carbonate examples locally contain evidence of subaerial exposure and meteoric diagenesis. Commonly, much of the porosity formed during exposure is ultimately reduced by compaction and cementation during early burial. By contrast, no evidence of meteoric diagenesis is observed in associated basinal carbonates, although compaction and cementation accompanying progressive burial are readily evident. In both cases, however, such early diagenesis is overprinted by late burial dissolution, sometimes coincident with hydrocarbon emplacement, creating rocks of high porosity. The formation of porosity by cement dissolution may exhume occluded pores or enhance relict pores that formed in the eogenetic zone, the result being a preponderance of interparticle and moldic pores and residual cements that mimic vadose and phreatic products. In other cases, nonfabric selective dissolution, locally associated with fractures or stylolites, creates vuggy porosity which may resemble that formed during eodiagenesis. Multiple phases of deep-burial dissolution and partial cementation or replacement (by calcite or dolomite) are indicated for many of these diagenetic systems and result in a complex suite ofmore » different pore types.« less

Marine Diagenesis on Enewetak Atoll: ABSTRACT

Marine waters currently circulate through Eocene and Miocene limestones deposited in slope, fore-reef, and reef-related environments on Enewetak Atoll. Subsurface core samples (375-1,400 m deep) demonstrate that marine waters have extensively altered the original carbonate sediments. Evidence supporting alteration by marine water includes: (1) calcite cements with isotopic compositions characteristic of a marine origin (^dgr13C = 1.3 to 2.5^pmil; ^dgr18O = -1.8 to 0.4^pmil PDB), (2) dolomites with average isotopic compositions that are in equilibrium with cold normal-marine waters (^dgr13C = 2.3^pmil; ^dgr18O = 2.5^pmil PDB), (3) consistent magnesium concentrations in fossil coralline algae, and (4) a lack of diagnostic evidence for meteoric diagenesis. Products of marine diagenesis on Enewetak Atoll are dependent on the CaCO3 saturation state of the diagenetic waters. Above the aragonite saturation depth (350 m), marine diagenesis is dominated by aragonite and high-magnesian-calcite cementation. Between aragonite and calcite saturation depths, marine waters apparently have dissolved aragonite and precipitated low-magnesian, radiaxial calcite cements. Below the calcite saturation depth (1,000 m), deep marine waters have apparently dissolved some calcite and have precipitated dolomite. Effective marine diagenesis requires limestones with good permeability and a hydraulic drive, such as tidal pumping or thermal convection, to move large volumes of marine water through the rock. End_of_Article - Last_Page 304------------