Friday, November 30, 2012

Highlights—Rivers on the Down Low(stand)

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.

Thursday, November 29, 2012

Highlights—'N Synch in the Paradox

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.  

Friday, November 16, 2012

A Look Back…80 years ago: Microbial Carbonates

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….”

Thursday, November 15, 2012

A Look Back…75 years ago: Microbial Lacustrine Carbonates

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.

Wednesday, November 14, 2012

A Look Back…50 years ago: The Evolving Science of Stratigraphy

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.

Thursday, November 8, 2012

Highlights – Not All Mud Is Transported Equally...

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.