Michael C. Pope

Global Events Across the Mesoproterozoic-Neoproterozoic Boundary: C and Sr Isotopic Evidence from Siberia

Thick, unmetamorphosed successions of siliciclastic and carbonate rocks in eastern and western Siberia preserve a record of Middle Riphean to Early Upper Riphean sedimentary environments and geochemistry. Consistent with data from other continents, our studies in the Uchur–Maya region in southeastern Siberia and the Turukhansk Uplift in northwestern Siberia suggest a first-order shift in 13 C from values near 0‰ in the early Mesoproterozoic to values near +3.5‰ after about 1300 Ma. Over this same interval, primary 87 Sr/ 86 Sr values decrease from 0.7060 to 0.7053. Combining lithologic, biostratigraphic, and geochemical data sets with available geochronologic constraints, we present a refined correlation between these two key Proterozoic successions in Siberia and add this dataset to a growing body of C and Sr isotopic data from this time interval. Carbon isotope chemostratigraphy from these regions supports the occurrence and timing of a first-order, 3.5‰ positive shift ca. 1250–1300 Ma, approximately coeval with the onset of active margin activity that predates the main phase of Rodinia assembly. Sr isotopic data may also be interpreted within the context of the evolving Mesoproterozoic tectonic regime. Available data suggest that no dramatic rise in 87 Sr/ 86 Sr heralds the main phase of Rodinia assembly in the terminal Mesoproterozoic, suggesting that significant juvenile crust was involved in mountain building, that relative hydrothermal flux from mid-ocean ridges remained high throughout the assembly of Rodinia and/or that increased continental runoff related to intense erosion of Grenvillian mountain belts terminated shortly after orogeny.

Widespread, prolonged late Middle to Late Ordovician upwelling in North America: A proxy record of glaciation?

Late Middle to Late Ordovician subtidal ramp carbonates of New Mexico, Texas, and Oklahoma contain abundant spiculitic chert (to 70% chert by volume) and phosphate (1–5 wt%), indicating that these rocks formed in an extensive upwelling zone. Upwelling began in the late Middle Ordovician (ca. 454 Ma) and persisted until the end of the Ordovician. Late Ordovician cherty carbonates also occur along the U.S. Cordilleran margin, in board of organic-rich graptolitic shale and chert. The widespread occurrence of Late Ordovician cherty and phosphatic carbonates on southern and western Laurentia, in addition to phosphate-rich, cool-water carbonates over much of the North American Midcontinent, suggests vigorous thermohaline circulation related to prolonged (10–14 Ma) Gondwana glaciation.

A Vendian-Cambrian Boundary Succession from the Northwestern Margin of the Siberian Platform: Stratigraphy, Palaeontology, Chemostratigraphy and Correlation

Siberia contains several key reference sections for studies of biological and environmental evolution across the Proterozoic-Phanerozoic transition. The Platonovskaya Formation, exposed in the Turukhansk region of western Siberia, is an uppermost Proterozoic to Cambrian succession whose trace and body fossils place broad limits on the age of deposition, but do not permit detailed correlation with boundary successions elsewhere. In contrast, a striking negative carbon isotopic excursion in the lower part of the Platonovskaya Formation permits precise chemostratigraphic correlation with upper-most Yudomian successions in Siberia, and possibly worldwide. In addition to providing a tool for correlation, the isotopic excursion preserved in the Platonovskaya and contemporaneous successions documents a major biogeochemical event, likely involving the world ocean. The excursion coincides with the palaeontological breakpoint between Ediacaran- and Cambrian-style assemblages, suggesting a role for biogeochemical change in evolutionary events near the Proterozoic Cambrian boundary.

Late Middle to Late Ordovician seismites of Kentucky, southwest Ohio and Virginia: Sedimentary recorders of earthquakes in the Appalachian basin

Synsedimentary ball-and-pillow beds, breccias, and faults in late Middle to Late Ordovician foreland basin rocks of Kentucky, southwest Ohio, and Virginia indicate broad zones of seismicity near the Cincinnati arch and Taconic orogenic front during deposition. Earthquake-induced liquefaction formed seismites, that include ball-and-pillow beds and rare sedimentary breccias that are correlative over large areas (hundreds to thousands of square kilometers). Comparison of these features with liquefaction structures in Holocene sediments indicates that the Ordovician ball-and-pillow beds were probably generated by large earthquakes (magnitudes >6). The Ordovician seismites also provide information about epicenter location and the recurrence interval of large earthquakes in the Ordovician foreland basin. Some were produced by faulting in the foreland and accretionary prism. However, horizons of resedimented lithoclastic breccias in the Jeptha knob cryptoexplosive structure appear to correlate with several ball-and-pillow beds on Jessamine dome, along the Cincinnati arch, suggesting that some of the seismites may be genetically related to this enigmatic structure.

Depositional history of pre-Devonian strata and timing of Ross orogenic tectonism in the central Transantarctic Mountains, Antarctica

A combination of field mapping, detailed sedimentology, carbon isotope chemostratigraphy, and new paleontological finds provides a significantly improved understanding of the depositional and tectonic history of uppermost Neoproterozoic and lower Paleozoic strata of the central Transantarctic Mountains. On the basis of these data, we suggest revision of the existing stratigraphy, including introduction of new formations, as follows. The oldest rocks appear to record late Neoproterozoic deposition across a narrow marine margin underlain by Precambrian basement. Siliciclastic deposits of the Neoproterozoic Beardmore Group—here restricted to the Cobham Formation and those rocks of the Goldie Formation that contain no detrital components younger than ca. 600 Ma—occupied an inboard zone to the west. They consist of shallow-marine deposits of an uncertain tectonic setting, although it was likely a rift to passive margin. Most rocks previously mapped as Goldie Formation are in fact Cambrian in age or younger, and we reassign them to the Starshot Formation of the Byrd Group; this change reduces the exposed area of the Goldie Formation to a small fraction of its previous extent. The basal unit of the Byrd Group—the predominantly carbonate ramp deposits of the Shackleton Limestone—rest with presumed unconformity on the restricted Goldie Formation. Paleontological data and carbon isotope stratigraphy indicate that the Lower Cambrian Shackleton Limestone ranges from lower Atdabanian through upper Botomian. This study presents the first description of a depositional contact between the Shackleton Limestone and overlying clastic units of the upper Byrd Group. This carbonate-to-clastic transition is of critical importance because it records a profound shift in the tectonic and depositional history of the region, namely from relatively passive sedimentation to active uplift and erosion associated with the Ross orogeny. The uppermost Shackleton Limestone is capped by a set of archaeocyathan bioherms with up to 40 m of relief above the seafloor. A widespread phosphatic crust on the bioherms records the onset of orogenesis and drowning of the carbonate ramp. A newly defined transitional unit, the Holyoake Formation, rests above this surface. It consists of black shale followed by mixed nodular carbonate and shale that fill in between, and just barely above, the tallest of the bioherms. This formation grades upward into trilobite- and hyolithid-bearing calcareous siltstone of the Starshot Formation and alluvial-fan deposits of the Douglas Conglomerate. Trilobite fauna from the lowermost siltstone deposits of the Starshot Formation date the onset of this transition as being late Botomian.

Ordovician metre-scale cycles: implications for climate and eustatic fluctuations in the central Appalachians during a global greenhouse, non-glacial to glacial transition

Abstract Metre-scale shallowing-upward cycles in Ordovician carbonates of the central Appalachian Basin record climate and eustatic fluctuations during a transition from Early Ordovician global greenhouse to Late Ordovician icehouse conditions. Peritidal facies and shale abundance suggest a long-term trend in this area from semi-arid (Early Ordovician) to more humid (Middle to early Late Ordovician) conditions, with a return to semi-arid conditions during the Late Ordovician. The climatic fluctuations were most likely produced by tectonics (uplift and erosion) related to Taconic orogenesis, plate motion of North America and the areal extent of water covering the shelf. Peritidal cyclic facies indicate that high-frequency relative sea level fluctuations were of small amplitude ( 20 m) relative sea-level fluctuations that decreased into the later Ordovician. If these sea-level changes are eustatic, then the increased amplitude may mark the early initiation of continental glaciation on Gondwana in the late Middle Ordovician, followed by waning of ice sheets prior to the latest Ordovician glaciation.

Paleoproterozoic Stark Formation, Athapuscow Basin, Northwest Canada: Record of Cratonic-Scale Salinity Crisis

ABSTRACT The Paleoproterozoic Stark Formation, a thick breccia unit, formed during the transition from marine to non-marine foreland basin fill in the Athapuscow Basin, Northwest Territories, Canada. Four types of breccia occur in the Stark Formation: (1) bedded breccias at the base of the unit, overlying the basinal facies of the underlying Pethei Group, are interpreted as debris flows, (2) massive chaotic breccias occur in the lower part of the Stark Formation throughout the basin and these are interpreted to be evaporite-collapse breccias; (3) siltstone and mudstone breccia occurs in the upper part of the Stark Formation and is also interpreted as evaporite collapse breccia, though much less evaporite was involved in its formation than the underlying chaotic breccia; and (4) laminar accretionary breccia consisting of laminar carbonate clasts, commonly with flat bottoms and convex-up upper surface geometries; these clasts occur in lenticular pods and are interpreted to form in caves following dissolution of evaporites or carbonates. The widespread presence and abundance of evaporite pseudomorphs (especially halite) in all breccia types, the stratigraphic restriction of the Stark Formation, and irregular filling above this unit indicates that all but the bedded breccia formed by the dissolution and collapse of evaporites. Chaotic breccia directly above underlying Pethei Group shallow-water and deep-water carbonates indicates the evaporates formed across the platform and adjacent deep basin. Foundered megaclasts (up to 1.5 km long and 40 m thick) in basinal settings suggest cumulative evaporite thicknesses on the order of a few tens to a few hundreds of meters. Carbonate clasts in all the breccias contain abundant wave ripples, planar laminations, stromatolites, and ooids. These structures along with a paucity of subaerial exposure features indicates nearly all carbonate deposition in the Stark was subaqueous. Thus, by inference the evaporites in the Stark Formation were also partially subaqueous. Displacive halite pseudomorphs are common throughout all facies in the Stark Formation. Silicified and dolomite-filled halite pseudomorphs are common along the contact between the evaporite-collapse breccia and underlying carbonate platform (Pethei Group) and surrounding large clasts in basinal settings. Silicified hopper crystal casts within deep-water rhythmites of the uppermost Pethei Group indicate precipitation of halite over deep waters. Gypsum pseudomorphs are exceedingly rare, and anhydrite pseudomorphs were not observed. The dominance of halite features in this basin and two penecontemporaneous basins (Kilihigok Basin and Wopmay Orogen) surrounding the Slave Craton suggests that, relative to the present day, seawater at 1.9 to 1.8 Ga was oversaturated with respect to calcium carbonate, had higher HCO3- concentration, and possibly was depleted in SO42-. The dearth of extensive, thick-bedded marine gypsum in the Archean and early Paleoproterozoic can be explained by low atmospheric oxygen content prior to 2 Ga in conjunction with increased carbonate saturation before 1.6 Ga.

NEW CHANCELLORIIDS FROM THE EARLY CAMBRIAN SEKWI FORMATION WITH A COMMENT ON CHANCELLORIID AFFINITIES

Abstract Articulated scleritomes of the chancelloriids Archiasterella fletchergryllus new species and Chancelloria cf. eros Walcott, 1920 are described from the Early Cambrian (Branchian) Sekwi Formation, Mackenzie Mountains, Northwest Territories, Canada. Early diagenetic, microbially mediated lithification has resulted in unusual three-dimensional preservation of the body surface, which potentially allows consideration of the evolutionary affinities of these enigmatic organisms. Sclerites are mounted on short stalks of the integument, connected to the undersurface of the central disc, and are external to the body surface.

Orbital-scale climate change and glacioeustasy during the early Late Ordovician (pre-Hirnantian) determined from δ18O values in marine apatite

This study focuses on the ∼10 m.y. before the latest Ordovician (Hirnantian) glaciation; we test whether orbital-scale climatic fluctuations controlled the growth and melting of continental glaciers, resulting in glacioeustatic sea-level changes and the development of widespread marine sedimentary cycles. δ18O values of conodont apatite from 14 Late Ordovician (Katian) cycles range from ∼17‰ to 21‰. Isotopic values decrease and are lowest in the deepest water facies and increase and are highest in shallow-water facies, supporting the hypothesis that glacioeustasy was the dominant control on water-depth changes. Measured intracycle δ18O changes of 0.7‰–2.5‰ were controlled by changes in ice volume (<60 m sea-level changes), sea-surface temperatures (<5 °C), and potentially local increases in seawater evaporation during drier and/or windier glacial stages. These interpreted orbital-scale climate changes and resultant large glacial ice-volume changes support recent interpretations of a dynamic and prolonged Ordovician greenhouse to icehouse transition.

Cassiar platform, north-central British Columbia: A miogeoclinal fragment from Idaho

The allochthonous Cassiar platform, in north-central British Columbia, is a cratonal fragment of ancestral North America juxtaposed against autochthonous North American crust along the Tintina–Northern Rocky Mountain trench fault. The Cassiar platform records a Neoproterozoic to early Paleozoic rift to passive-margin history that includes Lower Cambrian archeocyathan-bearing limestones of the Rosella Formation in the Cassiar Mountains. This study indicates that an extensive oolitic shoal developed toward the western edge of this carbonate platform during the deposition of the Nevadella zone, parallel to the western limit of thick continental crust (initial-Sr 0.706 isopleth). Paleogeographic studies from other archeocyathan-bearing units in the Cordillera indicate that a semicontinuous oolitic shoal was along the western margin of the continental shelf from Alaska to Mexico. There is a distinctive gap in the passive-margin record from southeastern Washington to southern Idaho. Paleogeographic constraints from the Rosella Formation and published paleomagnetic data from the overlying Sylvester allochthon suggest that this miogeoclinal slice was originally deposited near present-day Idaho and was transported northward, along poorly constrained dextral strike-slip faults.