Rapid sea level rise in the aftermath of a Neoproterozoic snowball Earth

Earth’s most severe climate changes occurred during global-scale “snowball Earth” glaciations, which profoundly altered the planet’s atmosphere, oceans, and biosphere. Extreme rates of glacioeustatic sea level rise are predicted by the snowball Earth hypothesis, but supporting geologic evidence has been lacking. We use paleohydraulic analysis of wave ripples and tidal laminae in the Elatina Formation, Australia—deposited after the Marinoan glaciation ~635 million years ago—to show that water depths of 9 to 16 meters remained nearly constant for ~100 years throughout 27 meters of sediment accumulation. This accumulation rate was too great to have been accommodated by subsidence and instead indicates an extraordinarily rapid rate of sea level rise (0.2 to 0.27 meters per year). Our results substantiate a fundamental prediction of snowball Earth models of rapid deglaciation during the early transition to a supergreenhouse climate

Rapid sea level rise in the aftermath of a Neoproterozoic snowball Earth

Earth’s most severe climate changes occurred during global-scale “snowball Earth” glaciations, which profoundly altered the planet’s atmosphere, oceans, and biosphere. Extreme rates of glacioeustatic sea level rise are predicted by the snowball Earth hypothesis, but supporting geologic evidence has been lacking. We use paleohydraulic analysis of wave ripples and tidal laminae in the Elatina Formation, Australia—deposited after the Marinoan glaciation ~635 million years ago—to show that water depths of 9 to 16 meters remained nearly constant for ~100 years throughout 27 meters of sediment accumulation. This accumulation rate was too great to have been accommodated by subsidence and instead indicates an extraordinarily rapid rate of sea level rise (0.2 to 0.27 meters per year). Our results substantiate a fundamental prediction of snowball Earth models of rapid deglaciation during the early transition to a supergreenhouse climate

Deposits from Wave-Influenced Turbidity Currents: Pennsylvanian Minturn Formation, Colorado, U.S.A.

Turbidity currents generated nearshore have been suggested to be the source of some sandy marine event beds, but in most cases the evidence is circumstantial. Such flows must commonly travel through a field of oscillatory flow caused by wind-generated waves; little is known, however, about the interactions between waves and turbidity currents. We explore these interactions through detailed process-oriented sedimentological analysis of sandstone event beds from the Pennsylvanian Minturn Formation in north-central Colorado, U.S.A. The Minturn Formation exhibits a complex stratigraphic architecture of fan-delta deposits that developed in association with high topographic relief in a tectonically active setting. An ~20–35-m-thick, unconformity-bounded unit of prodelta deposits consists of dark green shale and turbidite-like sandstone beds with tool marks produced by abundant plant debris. Some of the sandstone event beds, most abundant at distal localities, contain reverse-to-normal grading and sequences of sedimentary structures that indicate deposition from waxing to waning flows. In contrast, proximal deposits, in some cases less than a kilometer away, contain abundant beds with evidence for deposition by wave-dominated combined flows, including large-scale hummocky cross-stratification. We interpret the majority of these event beds as a record of deposition from hyperpycnal flows, i.e., turbidity currents generated directly from highly concentrated river plumes, which accelerated and decelerated in response to a rising and falling flood discharge. Additional support for this interpretation includes the following: (1) a variety of sole marks including flute and gutter casts, as well as tool marks made by relatively large (up to tens of centimeters across) woody debris (i.e., groove, prod, and chevron marks); (2) consistent unimodal orientation of sole marks and abundant ripple cross-stratification, which indicate strong downslope-directed flow; (3) a well documented sedimentological framework for the formation of fan-delta deposits adjacent to nearby highlands; and (4) plant fossils typical of middle- to high-elevation habitats that are abundant in the turbidite beds but absent in underlying and overlying shoreline and marginal marine deposits, which have a separate floral assemblage. Differences in grain sizes, vertical stratification sequences, and bed thicknesses between outcrops are interpreted to result from the spatial distribution of wave effects, the time history of hyperpycnal flows, and the interaction of these processes. The latter varied both spatially and temporally and produced a wide range of bed types, which are incorporated into a new conceptual model for storm-influenced hyperpycnal flows

Reconciling Himalayan midcrustal discontinuities: The Main Central thrust system

The occurrence of thrust-sense tectonometamorphic discontinuities within the exhumed Himalayan metamorphic core can be explained as part of the Main Central thrust system. This imbricate thrust structure, which significantly thickened the orogenic midcrustal core, comprises a series of thrust-sense faults that all merge into a single detachment. The existence of these various structures, and their potential for complex overprinting along the main detachment, may help explain the contention surrounding the definition, mapping, and interpretation of the Main Central thrust. The unique evolution of specific segments of the Main Central thrust system along the orogen is interpreted to be a reflection of the inherent basement structure and ramp position, and structural level of exposure of the mid-crust. This helps explain the variation in the timing and structural position of tectonometamorphic discontinuities along the length of the mountain belt

Geobiology of a lower Cambrian carbonate platform, Pedroche Formation, Ossa Morena Zone, Spain

The Cambrian Pedroche Formation comprises a mixed siliciclastic-carbonate succession recording subtidal deposition on a marine platform. Carbonate carbon isotope chemostratigraphy confirms previous biostratigraphic assignment of the Pedroche Formation to the Atdabanian regional stage of Siberia, correlative to Cambrian Series 2. At the outcrop scale, thrombolitic facies comprise ~. 60% of carbonate-normalized stratigraphy and coated-grains another ~. 10%. Petrographic point counts reveal that skeletons contribute at most 20% to thrombolitic inter-reef and reef-flank lithologies; on average, archaeocyath clasts make up 68% of skeletal materials. In contrast, petrographic point counts show that skeletons comprise a negligible volume of biohermal and biostromal thrombolite, associated nodular carbonate facies, and ooid, oncoid and peloid grainstone facies. As such, archaeocyathan reefal bioconstructions represent a specific and limited locus of skeletal carbonate production and deposition. Consistent with data from coeval, globally dispersed lower Cambrian successions, our analysis of the Pedroche Formation supports the view that lower Cambrian carbonates have more in common with earlier, Neoproterozoic deposits than with younger carbonates dominated by skeletal production and accumulation. © 2013 Elsevier B.V.Jessica R. Creveling, David Fernández-Remolar, Marta Rodríguez-Martínez, Silvia Menéndez, Kristin D. Bergmann, Benjamin C. Gill, John Abelson, Ricardo Amils, Bethany L. Ehlmann, Diego C. García-Bellido, John P. Grotzinger, Christian Hallmann, Kathryn M. Stack, Andrew H. Knol

Origin of giant wave ripples in snowball Earth cap carbonate

The most extreme climate transitions in Earth history are recorded by the juxtaposition of Neoproterozoic glacial deposits with overlying cap carbonate beds. Some of the most remarkable sedimentary structures within these beds are sharp-crested (trochoidal) bedforms with regular spacing of as much as several meters that are often interpreted as giant wave ripples formed under extreme wave conditions in a nonuniform postglacial climate. Here we evaluate this hypothesis using a new bedform stability diagram for symmetric oscillatory flows that indicates that the first-order control on the formation of trochoidal rather than hummocky bedforms is sediment size, not wave climate. New measurements of bedform wavelengths and particle sizes from the ca. 635 Ma Nuccaleena Formation, Australia, indicate that the giant ripples are generally composed of coarse to very coarse sand; most are within the trochoidal bedform stability phase space for normal wave climates. Moreover, numerical simulations of flow over fixed bedforms show that symmetric trochoidal ripples with a nearly vertical angle of climb may be produced over long time periods with variable wave climates in conjunction with rapid seabed cementation. These data reveal that, rather than extreme wave conditions, the giant wave ripples are a consequence of the unusual mode of carbonate precipitation during a global carbon cycle perturbation unprecedented in Earth history