Many
hydrocarbon-bearing reservoirs include cyclic strata, and understanding the
controls can provide important predictive insights. In this study, Sullivan and Sullivan examine the
unconformity-bounded estuarine and fluvial sandstones of middle Eocene Domengine
Formation of the Sacramento basin, California. The results illustrate that tectonism controlled the location
of incised fluvial and estuarine systems that stack vertically and trend
southwest toward the structurally controlled depocenter, but that eustasy
controlled the timing of the regressive-transgressive depositional cyclicity. These results provide a conceptual
model for architecture, thickness trends, and facies distribution in this, and
perhaps other, lowstand river-dominated estuarine units.
As environments near mean sea level, mixed siliciclastic-carbonate shallow marine and coastal systems are sensitive to both relative changes in sea level and climate shifts. Jordan and Mountney document facies, stratigraphy, and cyclicity in the Pennsylvanian-Permian lower Cutler beds of the Paradox Basin, USA, to explore variability in contemporaneously active eolian, fluvial, and shallow marine systems. The results indicate that relative changes in sea level and climate shifts were linked, and how each of these distinct subenvironments responded to these changes. The data provide a conceptual model for stratigraphic architecture within and among cycles and, based on these insights, offers a conceptual model for correlation strategies that should be applicable to other mixed systems.
Although
application of new tools to study microbial processes have revealed novel
insights into the role of microbes on calcium carbonate precipitation and
accumulation of many limestone successions, the role of micro-organisms was
postulated long ago. For example,
eighty years ago, Gee reviewed the
observations and interpretations of the role of bacterial activity on the
character and accumulation of carbonates.
The review suggested that “…the inferences have usually outrun the
established facts.” He surmised
that “biological and the chemical aspects of this geological problem can
therefore not be considered as distinct from one another,” but that at least in
some settings, the biologic component may be the limiting factor. The study cautioned, however, that “sulphur
organisms, with endocellular calcareous granules, are the only bacteria to
which the phrase ‘specific precipitating power’ applies….”
Although
recent discoveries and exploitation in the South Atlantic have re-emphasized
their importance, for many years, lacustrine carbonates have been de-emphasized
relative to their more abundant open marine counterparts. Seventy-five years ago, Twenhofel described bottom sediment in Lake Monona in Wisconsin. The study of cores from this lake revealed
black organic-rich sludge underlain by lighter-colored firm marls. Perhaps heralding recent emphasis into
the importance of microbial processes in lacustrine carbonates, the results
were interpreted to reflect the importance of bacteria in degradation of
organic matter, precipitation of calcium carbonate, and, ultimately, the manner
and style of accumulation of these lake deposits.
For many
years, the field of stratigraphy was largely a descriptive science. In his address as president of SEPM in
1962, Sloss suggested that the aspects of stratigraphy which
addressed the “patterns in space and time formed by the bodies of rock” that
form sedimentary rocks remained poorly understood due in part because of the
“awesome complexities involved.”
He advocated an approach in which stratigraphy might become more
predictive and quantitative, facilitated by “analysis of sedimentologic data in
terms of specific process variables which interact to produce an observable
stratigraphic response,” such as a distinct geometry or composition.
Understanding the genesis of muddy successions, although widespread and stratigraphically and economically important, has lagged behind that of its coarser counterparts. In this paper, Plint et al. address the question of how was mud transported for more than 200 km across a shallow, low-gradient ramp in the Upper Cretaceous foreland basin of Alberta, Canada? Microscopic examination shows that organic-rich mudstone (up to 11% TOC; a major source-rock) was transported mainly in the form of low density silt-size aggregates of clay mineral grains. Aggregates are relatively well consolidated and are interpreted to have been reworked from shallowly-buried (cm-dm) sediment and transported as bedload by storms-driven combined and geostrophic flows on this extremely low-gradient ramp. These results illustrate a means by which clay mineral-rich sediment, far removed from their source, can accumulate in relatively shallow water and preserve organic matter.
