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R.M. McKay

Publications and source records attributed to R.M. McKay.

6 recordsLinked to original sources
USGS2011

Oxygen-isotope trends and seawater temperature changes across the Late Cambrian Steptoean positive carbon-isotope excursion (SPICE event)

The globally recognized Late Cambrian Steptoean positive C-isotope excursion (SPICE) is characterized by a 3‰–5‰ positive δ 13 C shift spanning <4 m.y. Existing hypotheses suggest that the SPICE represents a widespread ocean anoxic event leading to enhanced burial/preservation of organic matter (C org ) and pyrite. We analyzed δ 18 O values of apatitic inarticulate brachiopods from three Upper Cambrian successions across Laurentia to evaluate paleotemperatures during the SPICE. δ 18 O values range from ∼12.5‰ to 16.5‰. Estimated seawater temperatures associated with the SPICE are unreasonably warm, suggesting that the brachiopod δ 18 O values were altered during early diagenesis. Despite this, all three localities show similar trends with respect to the SPICE δ 13 C curve, suggesting that the brachiopod apatite preserves a record of relative δ 18 O and temperature changes. The trends include relatively high δ 18 O values at the onset of the SPICE, decreasing and lowest values during the main event, and an increase in values at the end of the event. The higher δ 18 O values during the global extinction at the onset of the SPICE suggests seawater cooling and supports earlier hypotheses of upwelling of cool waters onto the shallow shelf. Decreasing and low δ 18 O values coincident with the rising limb of the SPICE support the hypothesis that seawater warming and associated reduced thermohaline circulation rates contributed to decreased dissolved O 2 concentrations, which enhanced the preservation/burial of C org causing the positive δ 13 C shift.

Geology↗
USGS2008

The record of time in cratonic interior strata: Does exceptionally slow subsidence necessarily result in exceptionally poor stratigraphic completeness?

A newly constructed a high-resolution chronostratigraphic and lithostratigraphic framework for the Upper Cambrian and Lower Ordovician Sauk Sequence in the cratonic interior of North America provides insight into the long-standing question of how time is recorded in sedimentary packages deposited in shallow epeiric seas across regions with exceptionally slow subsidence. It reveals that time is recorded in these strata in a manner fundamentally similar to the way it is in a number of nearshore marine-dominated sedimentary packages that were deposited under conditions of markedly higher subsidence rates. The principal consequence of slow subsidence in the cratonic interior appears largely to be a pronounced shingling of chronostratigraphic units perpendicular to depositional strike. An evaluation of relative stratigraphic completeness of the Upper Cambrian and Lower Ordovician of this region suggests that a number of routine interpretations and assumptions must be re-evaluated. Our results are inconsistent with the common interpretation that: (1) cratonic interior sedimentary packages are exceptionally stratigraphically incomplete; and (2) that conditions of very slow subsidence and a bathymetrically shallow shelf by themselves preclude deposition of a relatively complete record of time. In refuting these conventional assumptions, our conclusions have implications for a variety of approaches that require a fundamental understanding of the stratigraphic record of time, such as efforts to construct eustatic sea level curves and evaluations of the fossil record of evolution.

Conference Paper↗
USGS2007

High-resolution sequence stratigraphy of lower Paleozoic sheet sandstones in central North America: The role of special conditions of cratonic interiors in development of stratal architecture

Well-known difficulties in applying sequence stratigraphic concepts to deposits that accumulated across slowly subsiding cratonic interior regions have limited our ability to interpret the history of continental-scale tectonism, oceanographic dynamics of epeiric seas, and eustasy. We used a multi-disciplinary approach to construct a high-resolution stratigraphic framework for lower Paleozoic strata in the cratonic interior of North America. Within this framework, these strata proved readily amenable to modern sequence stratigraphic techniques that were formulated based on successions along passive margins and in foreland basins, settings markedly different from the cratonic interior. Parasequences, parasequence stacking patterns, systems tracts, maximum flooding intervals, and sequence-bounding unconformities can be confidently recognized in the cratonic interior using mostly standard criteria for identification. The similarity of cratonic interior and foreland basin successions in size, geometry, constituent facies, and local stacking patterns of nearshore parasequences is especially striking. This similarity indicates that the fundamental processes that establish shoreface morphology and determine the stratal expression of retreat and progradation were likewise generally the same, despite marked differences in tectonism, physiography, and bathymetry between the two settings. Our results do not support the widespread perception that Paleozoic cratonic interior successions are so anomalous in stratal geometries, and constitute such a poor record of time, that they are poorly suited for modern sequence stratigraphic analyses. The particular arrangement of stratal elements in the cratonic interior succession we studied is no more anomalous or enigmatic than the variability in architecture that sets all sedimentary successions apart from one another. Thus, Paleozoic strata of the cratonic interior are most appropriately considered as a package that belongs in a continuum of variable stratigraphic packages reflecting variable controls such as subsidence and shelf physiography. Special conditions of exceptionally slow subsidence rate, shallow bathymetry, and nearly flat regional shelf gradient are manifest mostly by the presence of individual systems tracts of relatively long duration that extend for much greater distances across depositional strike than those that characterize successions deposited in more dynamic tectonic and physiographic settings. These results suggest that if other cratonic interior successions are as anomalous as reported, a low sediment supply may have played a primary role in development of their apparently condensed stratal architecture. The results also lead us to suggest that a nonvegetated lower Paleozoic landscape played a relatively insignificant role in the development of what are commonly perceived to be enigmatic stratigraphic features of sheet sandstones, particularly their widespread yet thin geometry, and a scarcity of shale and siltstone. ?? 2007 Geological Society of America.

Geological Society of America Bulletin↗
USGS1998

Origin of a classic cratonic sheet sandstone: Stratigraphy across the Sauk II-Sauk III boundary in the Upper Mississippi Valley

The origin of cratonic sheet sandstones of Proterozoic and early Paleozoic age has been a long-standing problem for sedimentologists. Lower Paleozoic strata in the Upper Mississippi Valley are best known for several such sandstone bodies, the regional depositional histories of which are poorly understood. We have combined outcrop and subsurface data from six states to place the Upper Cambrian Wonewoc (Ironton and Galesville) Sandstone in a well-constrained stratigraphic framework across thousands of square kilometers. This framework makes it possible for the first time to construct a regional-scale depositional model that explains the origin of this and other cratonic sheet sandstones.

Geological Society of America Bulletin↗
USGS1988

A new Lower Carboniferous tetrapod locality in Iowa

The earliest tetrapods known are from two or three Upper Devonian 1–3 and some 20 Lower Carboniferous localities in Scotland4 and North America 5–8 . Most sites yield few and fragmentary specimens; well-preserved and even partially articulated material is exceedingly rare. This report discusses a middle Lower Carboniferous site rich in amphibian and fish remains discovered near Delta, south-east Iowa, and represents the first Lower Carboniferous tetrapod locality found in mid-continental North America. The bones occur within collapse-structures or depressions, and appear to represent a fresh- or brackish-water pond fauna. The Delta site contains the oldest well-preserved tetrapod fauna in North America, and one of the oldest in the world. Several hundred tetrapod fossils have been collected to date, with excavation somewhat more than half completed. Specimens range from isolated bones to articulated, nearly complete skeletons of at least two apparently new amphibian species. This material will make an important contribution to knowledge of the mor-phology and interrelationships of early tetrapods.

Iowa↗