Since the beginning of the century, several authors have hypothesized and documented the presence of bottom currents during the deposition of mudstones, including mudstones rich in organic matter, challenging the assumption that persistent low-energy conditions are necessary prerequisites for deposition of such sediments. More processes responsible for transport and deposition of mudstones mean also more processes acting contemporaneously in different parts of a basin. Without a precise and robust chronostratigraphic framework, however, it is not possible to characterize these differences. The new data reported here provide a profoundly different understanding of the controls on sedimentation in distal continental shelf platforms. To enhance the understanding of the different coeval environments of deposition coexisting in a muddy system, the Upper Cretaceous Eagle Ford Group, deposited on the Comanche carbonate platform, has been investigated by integrating sedimentology, mineralogy, geochemistry and palaeoecology, and creating age models in different physiographic sectors using biostratigraphy and geochronology. Data from two cores and 41 outcrops were analysed with a telescopic approach, from grain scale to basin scale. Nine temporal stages over a ca 8 Myr interval (ca 98 to 90 Ma) were defined in an area that spans 75 000 km 2 . Finally, the different environments of deposition recorded within each of the nine stages were interpreted. The construction of the chronostratigraphic framework also allowed: measuring the duration of a basin-wide gradational increase in energy in the water column (ca 1 Myr) and a hiatus confined into the shallower water sector (ca 2 Myr); determining the mean eruption frequency of volcanoes (ca 9 kyr); and the time of inundation of the Western Interior Seaway (97Á5 to 97Á1 Ma). The context, the outcropscores-logs correlations, the large data set (Appendix S1), the high-precision and well-calibrated constraints represent an unprecedented contribution for future regional facies models of organic-rich units and for improvements of key aspects in the industry of unconventional resources. Buda Limestone 83 m E. Cen. M. Cen. L. Cen. E. Tur. M. Tur. L. Tur. 3 4 5 8 1 6 7 9 50 m J D 65 m H C E 44 m J D 34 m Jnod CL 78 m M C G Conia. 1524 D. Minisini et al. 91 92 93 94 95 96 97 90 89 98 99 100 101 Age (Myr) 91 92 93 94 95 96 97 90 89 98 Lt M E CENOMANIAN TURONIAN Lt M E CONIACIAN
The Middle Triassic Latemar Platform is an approximately 5 km wide, 700 m thick isolated carbonate platform succession containing over 500 cycles (<1 m average thickness), which have been attributed to allocyclic forcing by Milankovitch to subMilankovitch cycle driven composite eustasy. These interpretations are based on the facies composition of the cycles (thicker subtidal units overlain abruptly by thin, centimetre-scale, subaerial caps), the 5:1 bundling of the 'fundamental' metre-scale cycles into lower-frequency 'megacycles', and spectral analyses of thickness and rank series showing conspicuous matches to predicted Milankovitch periodicities. The infl uence of periodic Milankovitch cycle-driven composite eustasy on the development of the Latemar cyclic succession has been questioned based on the dating of ashfall tuffs, palaeomagnetic analysis of the platform, and correlation to biostratigraphic markers in nearby basinal deposits. In order to test the interpretation of allocyclic forcing for Latemar cycles, the cyclic succession preserved at Mendola Pass (located 30 km northwest of Latemar Platform) was investigated where Latemar-equivalent cyclic platform interior strata are well exposed. The Mendola cycles (average 0.70 m per cycle) are also bundled into upward-thinning packages with an approximate 5:1 ratio. However, unlike the subtidal deposits immediately overlain by vadose diagenetic caps found in the Latemar succession, Mendola cycles consist of a mud-rich subtidal unit gradationally overlain by a cryptomicrobial (peritidal) laminite cap. A measured section from Mendola Pass of 36 cycles correlates biostratigraphically and statistically to a unique interval within the Latemar succession. Although laminite-capped cycles are often attributed to autocyclicity, the similarity of the stacking patterns of the Mendola cycles to those of Latemar support an allocyclic interpretation. In addition, depositional rates calculated from dated Holocene shallow-water carbonate facies equivalent to those at Latemar and Mendola Pass are shown to be consistent with Milankovitch or multimillennial periodicities, rather than millennial (1 kyr) cycle periods. Finally, the question of whether comparative sedimentology can be relied upon for the interpretation of relative cycle duration is considered, or if comparative sedimentology has reached its useful limit for identifying facies and depositional environments with respect to the Middle Triassic. It is concluded that statistical and biostratigraphic correlations, in addition to comparative sedimentology, indicate that the Latemar and Mendola cycles were deposited under the control of an allogenic forcing mechanism, and that this mechanism generated depositional cycles with multimillennial individual periodicities.
The Cenomanian–Turonian mass extinction (Oceanic Anoxic Event 2-OAE2) was a period of profound ecological change that is recorded in the sedimentary record in many locations around the globe. In this study, we provide a new and detailed account of repetitive changes in water column ecology by analyzing the organic geochemical record preserved within the OAE2 section of the Greenhorn Formation, Western Interior Seaway (WIS) of North America. Results from this study provide evidence that OAE2 in the WIS was the result of the cumulative effect of reoccurring environmental stresses rather than a single massive event. During OAE2, extreme variations in biotic composition occurred erratically over periods of several thousands of years as revealed by molecular fossil (biomarker) abundances and distributions calibrated to sedimentation rates. These cycles of marine productivity decline almost certainly had follow-on effects through the ecosystem and likely contributed to the Cenomanian–Turonian mass extinction. While the causes behind organic productivity cycling are yet unproven, we postulate that they may have been linked to repeated episodes of volcanic activity. Catastrophic volcanism and related CO2 outgassing have been interpreted as main drivers for OAE2, though this study provides new evidence that repetitive, punctuated environmental stresses were also important episodes within the anatomy of OAE2. Following OAE2, these cycles of productivity decline disappeared, and the WIS returned to conditions comparable to pre-OAE2 levels.
Oceanic Anoxic Event‐2 (OAE‐2; Cenomanian‐Turonian) is characterized by extensive deposition of organic carbon‐rich deposits (black shales) in ocean basins worldwide as result of a major perturbation of the global carbon cycle. While the sedimentological, geochemical, and paleontological aspects of deep water expressions of OAE‐2 have been intensively studied in the last few decades, much less attention has been given to the coeval shallow water deposits. In this study, we present the results of a detailed facies and petrographic (optical microscope and scanning electron microscopy) and geochemical studies (δ13Ccarb, δ 13C org, δ 15Nbulk, TOC, and Rock‐Eval pyrolysis) on two key shallow marine sections from the Apennine Carbonate Platform (ACP; Italy). Here a continuous record of shallow water carbonates through the OAE‐2 interval is preserved, offering the unique opportunity to document the archive of paleoenvironmental changes in a neritic setting, at a tropical latitude and far from the influence of a large continental block. Two conspicuous intervals are characterized by abundant and closely spaced “dark” microbial laminites found at correlative stratigraphic horizons in the two stratigraphic sections. These laminites contain elevated concentrations of TOC (up to 1%) relative to microbial capping cycles laminites stratigraphically above and below. The organic matter preserved in these fine‐grained laminites is dominated by cyanobacteria remains, which accounted for most of the organic matter produced on the ACP in these intervals. Our study suggests that Tethyan carbonate platforms experienced significant biological changes during OAE‐2, alternating, in few kiloyears, between eutrophic phases dominated by microbial communities and mesotrophic/oligotrophic conditions favoring “normal” carbonate production/sedimentation. The synchronous occurrence of microbialite facies at different locations across the ACP and on other platforms worldwide (e.g., Mexico and Croatia) suggests a causal link between Large Igneous Province volcanism and the environmental conditions necessary to trigger cyanobacterial proliferation on shallow carbonate platforms.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.