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.

Friday, January 23, 2015

A Look Back…25 Years: Ireland Ice Sheet Collapse

The nature and rate of decay of ice sheets is central to understanding possible impacts of climate change. Twenty-five years ago, McCabe and Dardis documented the character and distribution of Late Pleistocene drumlins, deposited in association with the melt of the ice sheet in western Ireland. The results illustrated several lithofacies associations, interpreted in the context of depositional environment in association with the ice sheet. These insights provided a conceptual model for ice-distal to ice-contact subaqueous deposition, modified by till deposition and drumlinization. The results provided a detailed documentation of rapid facies changes associated with a dynamic ice margin, and how these details “may not therefore be linked directly to climatic change.”

Tuesday, January 6, 2015

A Look Back…50 Years: Analcime in the Record

Sedimentary rocks are mineralogically diverse, the result of depositional and diagenetic variability. Fifty years ago, Teruggi reported a new occurrence of sedimentary analcime, which he suggested was “one of the most important so far recorded in the geological literature.” His results, based on the study of a single section, revealed “considerable quantities of sedimentary analcime” in Argentina. He interpreted the analcime to be a diagenetic alteration of “altered vitroclastic matrix.”