Monday, March 27, 2017

Highlights: Differences between the EARSs

The East African Rift System (EARS) includes a northern segment that is more volcanically influenced than regions to the south. To explore the roles of climate, tectonism, and volcanism and the architecture of sedimentary deposits in rift basins, this manuscript by Mtelela and others provides a sedimentologic investigation of the Pleistocene–Holocene upper Lake Beds, the uppermost stratigraphic unit exposed throughout the Rukwa Rift Basin in southwestern Tanzania. Integrating geologic mapping, lithofacies analysis, petrographic microscopy, scanning electron microscopy, and radiocarbon dating reveal alternating stacking patterns of landward- and basinward-stepping alluvial to fluvial channel, deltaic, and profundal lacustrine strata, bounded by unconformities. Sequence development in this rift basin is interpreted to be controlled largely by base-level changes drive by interplay between climate change and sediment supply, but was influenced by episodic volcanism in the Rungwe Volcanic Province.  Understanding these types of linkages is central to efforts in evaluating the resource potential of rift basins; in this basin, it also provides a foundational context for interpretation of regional paleoclimate and paleoenvironmental setting of vertebrate fossils.

Wednesday, March 22, 2017

Highlights: The Skinny on Limestone Micropores

Although most agree that micropores are important for scientific and economic reasons, geologists and material scientists have diverse opinions on the very definition of microporosity and its possible origin(s). To explore the nature and genesis of micropores, Hasiuk and others examine bulk stable-isotope and elemental analyses from a range of microporous limestone hydrocarbon reservoirs and a compilation of geochemical data from previous studies of subsurface microporous limestones from the Phanerozoic. Results illustrate microcrystals are composed of low-Mg calcite, and geochemical data suggest most are abiotic origin, but modified by shallow-burial diagenesis. On this basis, the authors suggest a uniform, diagenetic origin during early burial for the micrite that hosts most microporosity in limestone oil reservoirs. This understanding is a first step in modeling the distribution of microporosity in reservoirs, enhancing accurate prediction of reservoir performance.

Diagenetic origins of the calcite microcrystals that host microporosity in limestone reservoirs by Franek J. Hasiuk, Stephen E. Kaczmarek, and Shawn M. Fullmer

Monday, March 20, 2017

AE Interview: Get to Know Bruce Wilkinson

Bruce (Bieber?) Wilkinson, Department of Earth Sciences, Syracuse University

Q: What’s your research? 

A: Before its demise, I used to profess expertise in the general field of carbonate geology; often focused on making light of folk who worshiped at the church of our mother of divine parasequence. More recent efforts are directed toward subjects that are a bit more quantitative in flavor; my last paper was on trying to understand the “Sadler Effect” when pretending that rainfall is meteoric sediment. Today I finished up my input on using multidimensional scaling to understand differences among ages of zircons in terrigenous samples, and am working on a paper trying to derive intra-annual growth rates of clams from stable isotope profiles.

Q: Where is your favorite field area (and why)?

A: There are a bunch (yes, I go to the field; at least I used to). I guess if I had to choose, it would be the Miocene-Pliocene lake deposits along the southwestern margin of the Snake River plain. Thick and extensive lake-margin oolite and algal bioherms; hard to beat.

Q: What do you enjoy about serving as JSR AE? 

A: The demise of carbonate geology (it’s the reason Rankey has time to write a JSR blog); that “death” means much less work (I am damn near unemployed as a “carbonate AE” for the Journal.

Q: What was your favorite JSR paper from “back in the day” (or a recent year)?  

A: I guess it would be Bob Folk’s “The natural historyof crystalline calcium carbonate; effect of magnesium content and salinity”; the man rather invented the field of carbonate geology.

Q: What are your hobbies? 

A: We live on this 50-acre hobby farm in upstate New York; horses, goats, cats, turkeys, guinea fowl, pea fowl, chickens, ducks, geese, pigeons; love my John Deere tractor.

Q: What’s on your favorite Pandora station? 

A: I spend a few bucks on Spotify; I clamp on those Sony noise-canceling headphones, and try and do science to the likes of Tompall and the Glaser brothers, Justin Townes Earl, the Gourds, and Mason Porter. 

Thursday, March 16, 2017

Highlights: Connections and Transfers in Canyons

The heads of submarine canyons represent a critical link in the transfer of sediment from terrestrial sediment sources to deep basin sinks. Data from modern canyons and submarine fans suggests the strength of this connection determines the proportion and caliber of sediment stored in shelfal environments relative to that which is transferred to deep-water. Sweet and Blum document data on grain size, bathymetry and geochronology from twenty-four modern submarine canyons that demonstrate this link to be very sensitive to the distance between the canyon head and the shoreline, and, to a lesser extent, wave energy. A surprising observation from these data is the close relationship between the transport of grains of sand-size and coarser clasts and the distance between the river mouth or shoreline and the head of the submarine canyon. These data show the width of this zone filters the caliber of sediment delivered into deep-water with important implications for understanding sediment budgets and reservoir and seal distribution.

Monday, March 13, 2017

Highlights: Caldera Sediment

Although sedimentary deposits record the history of Earth-surface processes, only infrequently they are used to understand volcanic systems. In this paper, Murphy and others study the lacustrine sedimentary rocks deposited within the Long Valley Caldera, California, to understand the evolution of the caldera. The results reveal the nature of the volcaniclastic, chemical, and biogenic sediment, from near the caldera margin (Gilbert-type delta) to distal (diatomite and marl). The interpretations suggest that deformation, volcanism, catchment area shifts, and climate change each played distinct roles in controlling the nature and extent of sediment accumulations. These data and insights provide unique perspectives on this volcanic system, understandings distinct from those offered by igneous petrology or volcanology alone, and refine the conceptual model for sedimentary processes in caldera lakes in general.

Thursday, March 9, 2017

A Look Back: 60 Years—In Bars, Size Matters

Every student in undergraduate sedimentary geology class knows that fundamental descriptors of sediment include attributes of size and sorting. Examining a sand and gravel bar in the Brazos River near Hearne, Texas, this paper by Folk and Ward is the classic text for describing means to quantify these sedimentologic characteristics, and for showing the interpretive value of the data (as opposed to qualitative interpretation). The locals of Robertson County still enjoy their grains, as do geologists: “If one may be permitted to extrapolate from a small study such as this one and enter the seductive field of generalization,” the study ends one of the more penetrating conclusions in sedimentologic history: “Once a relationship is established in an ideal case, where the changes are laid out before the observer in their most perfect form, one soon learns to recognize the same relationships in less ideal examples, where the changes are obscure. The obscure examples, hitherto unfathomable, are explained in the light provided by the ideal examples. So it has been with the Brazos bar study.” The undergraduates memorizing Folk and Ward’s equations for skewness and kurtosis to impress at the party Friday night clearly are appreciative for the illumination, even if the utility of size is intuitively easier to grasp.

Monday, March 6, 2017

Highlights: The Rise and Fall of Tides

Tidal-dominated systems include complex simple and compound dunes, impacted by tidal and non-tidal processes. To better understand a channelized lower intertidal zone, Jo and Choi document an integrated multi-year morphodynamic and hydrodynamic data set from the macrotidal Gyeonggi Bay, Korea. The data provide a means to explore how external processes such as waves, and rain-induced discharge fluctuation shape intertidal compound dunes in time and space, controlling their stratigraphic architecture. The results of the study highlight the complexity of intertidal dune architecture, and provide criteria that could be used to distinguish between intertidal dunes and subtidal dunes in the stratigraphic record.