Thursday, March 12, 2015

Highlights—Abiotic Needle-Fiber Calcite

Needle-fiber calcite (NFC) is common diagenetic cement that has been found globally in many different types of limestone. The origin of NFC has been widely debated, and both abiogenic and biogenic processes have been invoked to explain its origin. In recent years, the origin of NFC has been closely allied to fungi. Jones and Peng document NFC in the spring deposit at Shiqiang, Yunnan Province, China, where NFC is common—but never associated with fungi. The study explores the different possible mechanisms and concludes that these NFC formed by abiogenic processes, driven largely by evaporation. These results highlight the critical problems in trying to ascertain the origin of carbonate cements, and the dangers of trying to ascribe all NFC to biogenic processes.

Monday, March 2, 2015

Highlights—Drainage Area of Distributive Fluvial Systems

By its very nature, the production of siliciclastic sediment destroys much of the direct evidence of its source region, so geoscientists are left with indirect tools to assess metrics such as catchment basin size. In this paper, Davidson and Hartley relate the areal extent of existing large (> 30 km in length) distributive fluvial systems (DFS) formed in endorheic basins to contributing drainage basin area. Regression analyses indicate a strong positive relationship between drainage area and DFS area; additionally, drainage basin relief influences sediment supply in terms of volume or caliber, which in turn affects the depositional gradient of the DFS surface and resultant channel planform. Application of these regression relationships to examples in the rock record shows that DFS area can be used as a proxy to predict the surface area of fluvially transported sediment deposited in a sedimentary basin from the contributing catchment. The modern regression relationships suggest a measurable link between source and sink in the sedimentary rock record, and provide a potential tool for more accurate prediction of preserved fluvial architecture within basin-scale climatic and tectonic contexts.

A quantitative approach to linking drainage area and distributive-fluvial-system area in modern and ancient endorheic basins by Stephanie K. Davidson and Adrian J. Hartley

Wednesday, February 25, 2015

Highlights—Goldilocks, Lucy, and Freshwater Limestones

Have you ever come home and found a blonde stranger sleeping in your just-right sized bed? The three bears did just that, and their intruder now has been immortalized in a recent paper by Ashley et al., who explore the conundrum of the occurrence freshwater limestones in arid rift basins. Documenting and interpreting the carbonate petrology in the context of well-documented geochronology and climate history of the East African Rift System reveals groundwater-fed limestone in wetlands, driven by precession-moderated cycles of precipitation, geologic structural controls (i.e., faults), playa flooding frequency and the general hydrology of the basin. These results implicate the multiple spatial and temporal controls on freshwater limestone, and illustrate how freshwater limestones require conditions have to be just right, i.e. the Goldilocks effect. Likewise, the interpretation of non-lacustrine carbonate suggests a new source of potable water for the early hominins of Olduvai Gorge.

Freshwater limestone in an arid rift basin: a Goldilocks effect by Gail M. Ashley, Carol B. de Wet, Manuel DomĂ­nguez-Rodrigo, Alyssa M. Karis, Theresa M. O’Reilly, and RoniDell Baluyot

Monday, February 23, 2015

Highlights—Tooth Extractions Expose Climate Trends

Orogenies can have a regional or even global influence on climate, but means to assess these influences can be masked in the stratigraphic record. Extracting oxygen isotopes from a diverse set of vertebrate phosphate materials, Suarez et al. explore the effect of the Sevier Orogeny on regional climate and paleohydrology, as recorded in the Cretaceous Cedar Mountain Formation. The results within the Cedar Mountain Formation suggest a regional rainshadow effect from the Sevier Orogeny. Compared to the average meteoric water values for this paleolatitude as determined by pedogenic carbonates, meteoric water values determined from crocodiles and turtles are within the range of values for 34°N paleo-latitude. The Sevier Orogeny had a major effect on the isotopic composition of river water, the result of significant depletion in runoff of high altitude water or snow. These sorts of analyses that distinguish between regional paleoclimatic effects of orogeny and those of global climatic factors are essential for evaluating perturbations to the global carbon cycle and the related changes in temperature (e.g. mid-Cretaceous global warming).

Thursday, February 12, 2015

Highlights—The Early Life of Unconventional Pores

In assessment of fine-grained unconventional reservoir rocks, discrimination of kerogen (primary, detrital organic matter (OM)) from bitumen (a diagenetic product of kerogen maturation) can be difficult, even at the SEM scale. To explore the nature of these systems, Milliken et al. describe observations of low-maturity, and presumably bitumen-free, samples of Pliocene-Pleistocene marine sapropels. The data illustrate the diversity of organic-matter-hosted pore systems, providing a useful baseline for identification of these fundamentally different OM types in mature OM-rich mudrocks, and highlighting some of the challenges. Knowing the proportion of detrital versus diagenetic organic matter in these types of mudrocks has significant implications for understanding the nature of petroleum systems in unconventional reservoirs.

SEM petrography of eastern Mediterranean sapropels: analogue data for assessing organic matter in oil and gas shales by Kitty L. Milliken, Lucy T. Ko, Maxwell Pommer, and Kathleen M. Marsaglia

Friday, February 6, 2015

Highlights—Cretaceous Source-to-Sink Mass Balance

The facies architecture of siliciclastic sedimentary systems is shaped by a range of controls. To assess the source-to-sink dynamics and controls on Cretaceous strata of the Western Interior Seaway, Hampson et al. examine the influence of sediment supply on stratigraphic architecture. Applying a new methodology of mass-balance analysis to a succession of linked alluvial-coastal-shelfal deposits, the paper quantifies downsystem-fining, sediment-partitioning and sediment-budget characteristics within the sediment routing systems of several sequences. As this work illustrates a novel approach to quantifying the relations between accommodation and sediment supply, parameters that are rarely constrained or quantified in conventional sequence stratigraphic interpretations, it holds the potential for new insights into stratigraphic forcing mechanisms.