Use of U–Pb ages of detrital zircons to infer maximum depositional ages of strata: A test against a Colorado Plateau Mesozoic database

Pelagic Cretaceous sediments, deposited in a range of palaeotectonic and palaeogeographic settings, from continents to oceans, are commonly black and bituminous. 3 particular time-envelopes define the major occurrences of such facies: late Barremian-Aptian-Albian, the Cenomanian-Turonian boundary and, to a lesser extent, the Coniacian-Santonian. These intervals define the duration of so-called Oceanic Anoxic Events during which global marine waters were relatively depleted in oxygen, and deposition of organic matter, derived from both terrestrial and planktonic sources, was widespread. Cretaceous OAEs correlate closely with transgressions, and such a correlation exists throughout the stratigraphical column. Flooding of land-masses is thought to have transported much terrestrial plant material seawards; the progressive increase in shelf-sea area is thought to have stimulated production of marine plankton. Bacterial consumption of this organic matter favoured the development of poorly oxygenated mid-to late Cretaceous waters in which many of the characteristic facies of the Period, including glauconitic sandstones and phosphatic chalks, were deposited.

Cretaceous anoxic events: from continents to oceans

Global warming is widely regarded to have played a contributing role in numerous past biotic crises. Here, we show that the end-Permian mass extinction coincided with a rapid temperature rise to exceptionally high values in the Early Triassic that were inimical to life in equatorial latitudes and suppressed ecosystem recovery. This was manifested in the loss of calcareous algae, the near-absence of fish in equatorial Tethys, and the dominance of small taxa of invertebrates during the thermal maxima. High temperatures drove most Early Triassic plants and animals out of equatorial terrestrial ecosystems and probably were a major cause of the end-Smithian crisis.

https://science.sciencemag.org/content/sci/338/6105/366.full.pdf

Lethally Hot Temperatures During the Early Triassic Greenhouse

Significant short-term carbon isotope fluctuations are present in Cretaceous pelagic limestones from widely distributed onshore sections in the Circum-Atlantic-western Tethyan region. More than 1,000 closely spaced samples were analyzed during this study. At least seven major ^dgr13C excursions can be correlated from section to section. The most important "heavy events" occur near the Aptian-Albian and Cenomanian-Turonian boundaries, whereas "light events" are near the Jurassic-Cretaceous, Albian-Cenomanian, Turonian-Coniacian, and Cretaceous-Tertiary boundaries. The association of "events" with stage boundaries and the consistent correlation of "events" between stratigraphic sections provides a significant new tool for time-rock correlation independent of stan ard biostratigraphic techniques. The temporal association of these carbon isotope events with stage boundaries (faunal and floral events), global eustatic sea-level variations, and oceanic "anoxic events" demonstrates the potential usefulness of carbon isotope studies in interpreting variations in paleo-oceanic circulation. Furthermore, the association of carbon isotope variations with anoxic events is potentially useful for evaluation of the precise timing and the magnitude of preservation of organic matter in deep-sea and continental-margin sediments. Thus, isotopic studies may aid in estimating potential hydrocarbon resources in largely unexplored oceanic basins or along continental margins.

Carbon Isotope Fluctuations in Cretaceous Pelagic Limestones: Potential Stratigraphic and Petroleum Exploration Tool

High-resolution carbon isotope measurements of multiple stratigraphic sections in south China demonstrate that the pronounced carbon isotopic excursion at the Permian-Triassic boundary was not an isolated event but the first in a series of large fluctuations that continued throughout the Early Triassic before ending abruptly early in the Middle Triassic. The unusual behavior of the carbon cycle coincides with the delayed recovery from end-Permian extinction recorded by fossils, suggesting a direct relationship between Earth system function and biological rediversification in the aftermath of Earth's most devastating mass extinction.

https://science.sciencemag.org/content/sci/305/5683/506.full.pdf

Large Perturbations of the Carbon Cycle During Recovery from the End-Permian Extinction

Four volcanic-ash beds bracket the Early-Middle Triassic boundary, as defined by conodont biostratigraphy, in a stratigraphic section in south China. High-precision U-Pb dates of single zircons allow us to place the Early to Middle Triassic (Olenekian-Anisian) boundary at 247.2 Ma. Magnetic-reversal stratigraphy allows global correlation. The new dates constrain the Early Triassic interval characterized by delayed biotic recovery and carbon-cycle instability to ∼5 m.y. This time constraint must be considered in any model for the end-Permian extinction and subsequent recovery.

Timing of recovery from the end-Permian extinction: Geochronologic and biostratigraphic constraints from south China

Permeabilities of Holocene carbonate sediments from Florida and the Bahamas, determined for 74 samples by water flow rates in a "variable-head" permeameter, range from 0.6 to 57,000 millidarcies. Porosities range from 40 to 78 percent. These parameters are related to depositional texture as follows: Table The muddiest sediments (i.e., the finest-grained) have the highest porosities but lowest permeabilities; this negative correlation between porosity and permeability is the reverse of the situation found in carbonate rocks, even as young as Pleistocene. High porosity and low permeability in sediments show a strong correlation with percentage of fines (<62 µm). From capillary pressure curves it is inferred that many of the pore throats of muddy carbonate sediments are less than 1 µm in diameter, at least after drying. Measured permeabilities are used to calculate cementation rates for simple models of upper phreatic zone cementation; the calculated rates would require excessive time to produce the degree of cementation seen in Late Pleistocene rocks of Florida and the Bahamas. The amounts of rainfall and evaporation are the most important factors in the degree of cementation.

Pore networks in Holocene carbonate sediments

Flow regimes in debris flow

The Great Bank of Guizhou (GBG) is an exceptionally well exposed isolated Triassic platform in the Nanpanjiang Basin of South China. The platform is exhumed with its depositional profile preserved and is dissected by a faulted syncline that exposes a complete and uncomplicated cross section providing a unique opportunity to evaluate mechanisms involved in its birth, evolution, and demise. The GBG formed near the southern margin of the Yangtze Platform during a deepening event that expanded the Nanpanjiang Basin and drowned the region surrounding the GBG in the latest Permian. Initial accumulation of the GBG began in the latest Permian with small reef mounds and open-marine skeletal packstones. Following the end-Permian extinction, cyanobacterial boundstones grew over the bank top. During the Early Triassic the GBG developed a low-relief bank profile with mobile oolite shoals at the margin, shallow-subtidal and peritidal deposits in the interior, and gentle slopes dominated by pelagics, debris-flow deposits, and turbidites at the basin margin. In the Middle Triassic (Anisian) the GBG developed a progressively steepening profile rimmed with massive Tubiphytes reefs. The platform was flat topped with tidal-flat deposits across the interior. Basin-margin deposition was dominated by turbidites and debris-flow deposits but eventually shifted to avalanche and rock-fall deposits as the slopes steepened to the angle of repose. In the Middle Triassic (Ladinian) an erosional escarpment up to 1700 m high developed at the margin. Platform-margin strata are bedded packstones similar to interior strata, whereas breccias at the basin margin contain coral-boundstone clasts suggesting erosion of reefs from the escarpment. A restricted subtidal lagoon formed in the interior, producing an atoll-like morphology. Later, a flat-topped profile was restored as tidal flats spread across the interior. In the beginning of the Late Triassic deepening contributed to termination of the GBG before siliciclastic turbidites and shales were deposited over the platform. In contrast with the well known platforms of the Dolomites of northern Italy, the GBG contains abundant muddy carbonates and a progressively steepening bank to reef-rimmed and escarpment architecture. The Italian platforms contain little mud and have angle-of-repose, pinnacle geometries. The GBG's larger size increased mud production and protected it from extensive winnowing of mud, which in turn resulted in off-bank shedding of muddy sediments that were stable on relatively gentle, basin-margin slopes which progressively steepened and ultimately led to avalanche deposits and a high-relief erosional escarpment. In contrast, the lesser mud content of the Dolomites platforms forced avalanche and talus deposition to dominate throughout deposition of basin margins, which in turn produced their angle-of-repose geometries.

Controls on Facies Architecture of a Large Triassic Carbonate Platform: the Great Bank of Guizhou, Nanpanjiang Basin, South China

Within a 5-year period two violent tropical storms, Hurricanes Donna and Betsy, passed over the Florida Keys. Although these hurricanes were of comparable size and intensity, their geologic effects differed. Both hurricanes caused extensive damage to the outer reefs, although Betsy acted essentially on a fauna from which Donna had removed the weaker elements. Large-scale movement of sand on White Bank which resulted from Hurricane Donna did not occur during Betsy. In Florida Bay, supratidal sedimentation on the islands and mainland was extensive during Donna, but virtually absent during Betsy. An exception to this was the deposition of spillover lobes of skeletal sand landward of the beach on Cape Sable. Unlike Donna, Hurricane Betsy passed over the northwest portion of the Great Bahama Bank. Large spillover lobes of oolitic sand were eroded back as much as 140 feet. Sedimentation on the Andros tidal flats was restricted to vegetated highs, even though the entire tidal flat complex was inundated by as muc...

https://www.jstor.org/stable/pdfplus/30066278.pdf

Paul Enos

Papers

Timing of recovery from the end-Permian extinction: Geochronologic and biostratigraphic constraints from south China

Pore networks in Holocene carbonate sediments

Flow regimes in debris flow

Controls on Facies Architecture of a Large Triassic Carbonate Platform: the Great Bank of Guizhou, Nanpanjiang Basin, South China

Hurricane Betsy in the Florida-Bahama Area: Geologic Effects and Comparison with Hurricane Donna