Joseph P. Smoot

Age of the Mono Lake excursion and associated tephra

Abstract The Mono Lake excursion (MLE) is an important time marker that has been found in lake and marine sediments across much of the Northern Hemisphere. Dating of this event at its type locality, the Mono Basin of California, has yielded controversial results with the most recent effort concluding that the MLE may actually be the Laschamp excursion (Earth Planet. Sci. Lett. 197 (2002) 151). We show that a volcanic tephra (Ash ♯15) that occurs near the midpoint of the MLE has a date (not corrected for reservoir effect) of 28,620±300 14C yr BP (∼32,400 GISP2 yr BP) in the Pyramid Lake Basin of Nevada. Given the location of Ash ♯15 and the duration of the MLE in the Mono Basin, the event occurred between 31,500 and 33,300 GISP2 yr BP, an age range consistent with the position and age of the uppermost of two paleointensity minima in the NAPIS-75 stack that has been associated with the MLE (Philos. Trans. R. Soc. London Ser. A 358 (2000) 1009). The lower paleointensity minimum in the NAPIS-75 stack is considered to be the Laschamp excursion (Philos. Trans. R. Soc. London Ser. A 358 (2000) 1009).

Sedimentary facies and depositional environments of early Mesozoic Newark Supergroup basins, eastern North America

Abstract The early Mesozoic Newark Supergroup consists of continental sedimentary rocks and basalt flows that occupy a NE-trending belt of elongate basins exposed in eastern North America. The basins were filled over a period of 30–40 m.y. spanning the Late Triassic to Early Jurassic, prior to the opening of the north Atlantic Ocean. The sedimentary rocks are here divided into four principal lithofacies. The alluvial-fan facies includes deposits dominated by: (1) debris flows; (2) shallow braided streams; (3) deeper braided streams (with trough crossbeds); or (4) intense bioturbation or hyperconcentrated flows (tabular, unstratified muddy sandstone). The fluvial facies include deposits of: (1) shallow, ephemeral braided streams; (2) deeper, flashflooding, braided streams (with poor sorting and crossbeds); (3) perennial braided rivers; (4) meandering rivers; (5) meandering streams (with high suspended loads); (6) overbank areas or local flood-plain lakes; or (7) local streams and/or colluvium. The lacustrine facies includes deposits of: (1) deep perennial lakes; (2) shallow perennial lakes; (3) shallow ephemeral lakes; (4) playa dry mudflats; (5) salt-encrusted saline mudflats; or (6) vegetated mudflats. The lake margin clastic facies includes deposits of: (1) birdfoot deltas; (2) stacked Gilbert-type deltas; (3) sheet deltas; (4) wave-reworked alluvial fans; or (5) wave-sorted sand sheets. Coal deposits are present in the lake margin clastic and the lacustrine facies of Carnian age (Late Triassic) only in basins of south-central Virginia and North and South Carolina. Eolian deposits are known only from the basins in Nova Scotia and Connecticut. Evaporites (and their pseudomorphs) occur mainly in the northern basins as deposits of saline soils and less commonly of saline lakes, and some evaporite and alkaline minerals present in the Mesozoic rocks may be a result of later diagenesis. These relationships suggest climatic variations across paleolatitudes, more humid to the south where coal beds are preserved, and more arid in the north where evaporites and eolian deposits are common. Fluctuations in paleoclimate that caused lake levels to rise and fall in hydrologically closed basins are preserved as lacustrine cycles of various scales, including major shifts in the Late Triassic from a wet Carnian to an arid Norian. In contrast, fluvial deposits were mainly formed in response to the tectonic evolution of the basins, but to some extent also reflect climatic changes. The Newark Supergroup illustrates the complexity of rift-basin sedimentation and the problems that may arise from using a single modern analog for sedimentary deposition spanning millions of years. It also shows that a tremendous wealth of depositional, climatic, and tectonic information is preserved in ancient rift-basin deposits which can be recovered if the depositional processes of modern rift-basin deposits are understood.

Anisotropy of magnetic susceptibility as a tool for recognizing core deformation: reevaluation of the paleomagnetic record of Pleistocene sediments from drill hole OL-92, Owens Lake, California

Abstract At Owens Lake, California, paleomagnetic data document the Matuyama/Brunhes polarity boundary near the bottom of a 323-m core (OL-92) and display numerous directional fluctuations throughout the Brunhes chron. Many of the intervals of high directional dispersion were previously interpreted to record magnetic excursions. For the upper ∼120 m, these interpretations were tested using the anisotropy of magnetic susceptibility (AMS), which typically defines a subhorizontal planar fabric for sediments deposited in quiet water. AMS data from intervals of deformed core, determined from detailed analysis of sedimentary structures, were compared to a reference AMS fabric derived from undisturbed sediment. This comparison shows that changes in the AMS fabric provide a means of screening core samples for deformation and the associated paleomagnetic record for the adverse effects of distortion. For that portion of core OL-92 studied here (about the upper 120 m), the combined analyses of sedimentary structures and AMS data demonstrate that most of the paleomagnetic features, previously interpreted as geomagnetic excursions, are likely the result of core deformation.

Volcanic rift margin model for the rift-to-drift setting of the late Neoproterozoic-early Cambrian eastern margin of Laurentia: Chilhowee Group of the Appalachian Blue Ridge

New data support a model of a volcanic rifted margin for eastern Laurentia and the breakup of the supercontinent Rodinia. Upper Neoproterozoic–lower Cambrian rocks of the Chilhowee Group in the Blue Ridge Province of eastern North America are subdivided into two facies assemblages separated by an unconformity. Historically, the rocks have been correlated as a tripartite division: 1) basal sandstone and conglomerate (Cochran, Unicoi, and Weverton Formations), 2) middle siltstone and shale (Nichols Shale, Hampton Shale, and the Hampton and Harpers Formations), and 3) an upper sandstone and shale (Nebo Quartzite, Murray Shale, Hesse Quartzite, and Helenmode, Erwin, and Antietam Formations). Sedimentary analyses show that boundaries of the newly defined facies assemblages transect the named stratigraphic units. Assemblage A consists of fluvial-lacustrine deposits with interbedded subaerial basalt flows overlain by marine deposits. Fluvial strata formed in anastomosing braided-river channels and are interbedded with mudstones deposited in shallow lacustrine plains. Overlying shallow marine deposits consist of upward-coarsening successions capped by over-thickened sandstones. Shale-rich turbidite deposits characterized by subdued bioturbation and common slump features occur at the top. Assemblage B consists of sandstone and shale deposited on a stable shelf. These upward-coarsening parasequences are stacked to form transgressive-regressive system tracts reflecting long-term sea-level fluctuations. A working hypothesis is that these rocks are comparable to those present in seismic reflection profiles and drill cores of the volcanic rift margins of the Atlantic and Indian Oceans. Assemblage A deposits formed during rapid subsidence associated with the formation of seaward-dipping reflectors. Flood basalts within fluvial sandstones are the landward facies of more voluminous basalt flows in the opening basin. Overlying marine deposits of Assemblage A were coeval with basaltic volcanism and deeper-water deposits to the east. Assemblage B unconformably overlies Assemblage A rocks and laps onto much older continental rocks. It was deposited on the craton into the late Cambrian. Much of the lateral lithologic variability and differences in character and thickness of the Chilhowee Group is attributed to the depositional setting that was influenced by the underlying rift architecture. It was further complicated by juxtaposition of rocks telescoped along numerous Paleozoic thrust faults. This model predicts that coeval rocks in outboard thrust sheets are finer-grained, mafic volcanic-rich marine sequences (Assemblage A), overlain by shelf-margin and basin deposits that lack volcanic rocks (Assemblage B). This model provides more refined sedimentary criteria for examination of other volcanic rift margins.

Abstracts of Other Conference Presentations

Many physical (structural and textural) and compositional (chemical and mineralogical) attributes of volcanic rocks are correlated with techtonic setting. However, only a few casual relations are known. Modern subduction-related volcanic rocks are persistently highly porphyritic and dominantly fragmental, especially if subaerial. These features are expected foe magmas rich in H2O, because ascending magmas become supercooled by effervescence leading to crystallization and explosive eruption. The high H2O is plausibly related to subduction of cool lithosphere rich in hydrous minerals. Has subducting lithophere always been cool and and hydrous? Deep submarine extrusions of identical magmas would be less porphyritic, less vesicular and not so explosive. Variable vesicularity and associated sulfide deposits could, however, help reveal original depths of extrusion. Magnesian olivineis stabilized by H2O in siliceous melts; consequently, olivine in blocky and/or fragmental (highly viscous) volcanic rocks in suggestive of a hydrous magma. Nearly all tholeiitic basalts crystallize cpx before opc at low pressures. Anomalous early opx in them seems best explained by siliceous contamination. But how do we account for Mauna Loa? Granitic contamination may deplete basalt in Na2O with consequent reversal of plagioclase zoning. Little altered oceanic ridge basalts have diagnostic chemical compositions, but most old rocks are strongly altered. Reclosure of the Atlantic would likely preserve parts of Iceland and Azores. How would we interpret them?