Monday, November 28, 2016

A Look Back: 25 Years—A (Pre-Facebook) Post on a Wall

The margins of carbonate platforms can be steep, and even form vertical walls. This contribution from Ginsburg et al. uses submersible observations of geomorphology, biota, and sediment (illustrated by striking hand-drawn figures) of one of the steepest margins, in Tongue of The Ocean (TOTO), a deep-water embayment in the Bahamas, to explore the growth potential of such margins. The results revealed a “luxurious reef-building community” including corals, sponges, and Halimeda present even on near-vertical walls; these organisms were interpreted to “produce slow but significant outward accretion of the wall.”  And so it turns out that beyond just being another brick in the wall, organisms are important in building these features.

The growth potential of a bypass margin, Great Bahama Bank by R.N. Ginsburg, P.M. Harris, G. Eberli, and Peter K. Swart

Tuesday, November 22, 2016

A Look Back—50 Years: Everyone Loves Red Beds

The origins of red beds have fascinated geoscientists for generations. In this contribution, Picard examined the Red Peak Member of the Chugwater Formation (Triassic) in west-central Wyoming, a thick succession of red sandstone and siltstone. Coupling field sedimentologic observations, petrography, X-ray diffraction, and well log analysis, the study focuses on the ubiquitous siltstone, rather than the sandstone units. The data led Picard to suggest that the climate was warm to hot and semi-arid to arid, but he ended with the penetrative suggestion that “Further study of [redbed] siltstones is urged.” Many geologists who favor gingers have gladly followed his advice.

Wednesday, November 16, 2016

Highlights—Deep-Water Chains

As study of the deep-water fans deposits has advanced to progressively finer scales, debate regarding the nature and origin of stacking patterns has become refined. To quantitatively test conceptual models, Terlaky and others statistically analyze stacking patterns of stratal elements in the Proterozoic Upper Kaza Group (Windermere Supergroup, Cariboo Mountains, southern Canadian Cordillera) by Markov-chain analysis, an under-utilized tool in deep-water sedimentologic research. The results of the analysis shows that the stacking pattern in this passive-margin sedimentary pile is statistically non-random, and is more heterogeneous than would be expected in a random distribution. These observations form the basis for a conceptual model for lobe development driven by splay deposition and frequent avulsion.  Beyond showing the utility of the tool, the results are interpreted to be significant for reservoir characterization in similar passive-margin settings, providing analogs and data on thicknesses, volumes, and connectivity of stratal elements and reservoir bodies.

Thursday, November 10, 2016

Highlights—Complexity in Tidally Modified Deltas

Regional sequence stratigraphic correlations of tidally-modified deltaic deposits are very difficult because of the spatial and temporal variability in facies distribution and resultant complex stacking patterns. To understand these types of systems, Burton and others examine the sedimentology and stratigraphy of the Loyd and Sego sandstones across the Uinta–Piceance basins by integrating outcrop and subsurface (core and log) data. The data lead to a sequence stratigraphic correlation and associated maps, from which they interpret controls on depositional processes over time. These types of well-constrained analogs can provide perspectives for enhanced interpretation of similar deposits in basins where subsurface data are sparse.

Monday, November 7, 2016

A Look Back: 10 Years—A History of Monotony, or Simply a Tedious Record?

Facies patterns of many carbonate depositional systems are described as complex.  In this contribution from 10 years ago, Beavington-Penney et al. describe another end member—the extremely homogenous (even boring) succession of the middle Eocene Seeb Formation in Oman.  Outcrop description, petrographic characterization, and biofacies analysis reveal a ~250 m thick succession of nodular, indistinctly bedded shallow-subtidal sediment, which is largely acyclic. These results were interpreted to highlight the role of bioturbation and rhizoturbation in homogenizing the succession, and destroying evidence for surfaces and horizons that represent “missing time.” The data and interpretations suggest that slow accumulation rates and intense sediment re-working drive homogeneity, and that it is not only unpredictability that can be monotonous.

Simon J. Beavington-Penney, V. Paul Wright, and Andrew Racey

Wednesday, November 2, 2016

Highlights—Paleosol Geochemistry and Paleoclimate: A Cautionary Tale

The geochemical attributes of paleosols have been used to estimate paleoenvironmental and paleoclimatic conditions through much of the Phanerozoic. Here, Michel et al. use plane-polarized and cathodoluminescence petrography, electron microprobe, and stable isotope ratio mass spectrometry observations of paleosols from Permo-Carboniferous strata of the Lodève Basin, France to evaluate the role of post-depositional alteration on geochemistry of paleosols. The results illustrate that macrofeatures within paleosols are retainend and can be used for paleoclimate reconstruction; however, geochemical signatures are neither reflective of soil formation nor appropriate to use in paleoclimate reconstruction due to diagenetic alterations. Yet, the paleosol carbonate cements and matrix clays preserve information that reflects a complicated calcite-cement stratigraphy recorded in nodules, and quartz, barite, dolomite, and Fe-rich calcite cementation. These results emphasize that pedogenic nodules, rhizoliths, and paleosol matrix should systematically evaluated for diagenetic alteration prior to applying geochemical based proxies to reconstruct paleoclimate.

Monday, October 31, 2016

Highlights—Paleoclimate and pCO2

Estimates of pCO2 in deep time provide fundamental constraints on ancient atmospheric conditions, yet can include considerable uncertainty because of variability related to distinct carbon sources. To better constrain the role of different OM sources on pCO2 estimates derived from pedogenic carbonate, Myers et al. use a range of sources of organic matter to estimate the concentration of carbon dioxide in the atmosphere in both modern and Jurassic soils. The results indicate that the pedogenic carbonate CO2 paleobarometer is sensitive to variation in stable carbon isotope composition of soil-respired CO2. Additionally, organic matter occluded in pedogenic calcite produces reasonable pCO2 estimates. 

Effects of different organic-matter sources on estimates of atmospheric and soil pCO2 using pedogenic carbonate by Timothy S. Myers, Neil J. Tabor, Louis L. Jacobs, and Robert Bussert

Thursday, October 27, 2016

Highlights—Quartz (hamster?) Transport and Surface Microtextures

Although provenance plays a fundamental role in establishing sandstone mineralogy, transport processes impact the sediment as well. To explore the potential utility of transport-associated microtextures, Sweet and Brannan analyze the abundance of glacially and fluvially induced microtextures to assess the role of fluvial overprint on glacially modified grains along ~188 km of the proglacial Chitina River, SE Alaska.  The analysis of SEM observations documents the relative proportions of glacially induced stylus microtextures (i.e., grooves, troughs, and gouges) and fluvial percussion-induced microtextures (i.e., v-shaped cracks and edge rounding) on quartz grain surfaces, and compares how this ratio changes with distance downstream. The results indicate that the glacially induced microtextures input to the river persist downstream, but are modified progressively by traction and saltation. This type of analysis could be applied to ancient fluvial strata provide insights to 1) differentiate between proglacial and nonglacial braided rivers and 2) reconstruct proglacial paleogeography from ancient strata. Furthermore, the results clearly show that it makes a difference how you roll.

Monday, October 24, 2016

Highlights—Imaging Cement Zonation

Many diagenetic events can be recorded in rocks as only subtle cement zones or corrosion surfaces on mineral faces. To better observe and characterize these features, Buckman and others illustrate the application of Charge Contrast Imaging (CCI) to examine diagenetic features including zoning, twinning, fractures, and geochemical changes. Applying the technique to a complex Cretaceous coquina carbonate and Devonian reef carbonate sample illustrates results comparable to SEM-Cathodoluminescence (and optical-CL), but with several advantages. 1) CCI does not suffer from image smearing, due to latency of the red light phase within CL, a different type of charge. 2) CCI typically is faster than SEM-CL. 3) CCI appears to highlight fabric and cement details within regions of carbonate that appear nonluminescent under CL. Nonetheless, as with cathodoluminescence, the specific character of CCI varies as a function of elemental composition and lattice dislocations.  

Charge contrast imaging (CCI): revealing enhanced diagenetic features of a coquina limestone by James O. Buckman, Patrick W.M. Corbett, and Lauren Mitchell