Monday, July 21, 2014

Highlights—Organic-Rich Mudstone: The Source of the Source

Many organic-rich mudstone deposits have been interpreted in the context of water column stratification. Here, Könitzer et al. examine temporal variations of, and controls on, the abundance and type of organic matter in late Paleozoic organic-rich mudstones from an epicontinental marine basin. Detailed microtextural analysis with data on total organic carbon (TOC) and bulk carbon isotope composition of organic material (δ13Corg) suggest alternative mechanisms for deposition of Lower Carboniferous succession. The data are interpreted to represent changes in productivity, variations in the delivery of siliciclastics and terrestrial organic matter. The abundance of organic matter stored in these extensive marine basins influenced the carbon cycle during this icehouse period in Earth history and the suitability of Late Mississippian mudstone successions of the U.K. as prospects for shale gas.



Tuesday, July 15, 2014

Highlights—Beach Ridges

Questions of how shorefaces respond to relative rises in sea level are of paramount importance for predicting morphosedimentary response to rising global sea level. In this study, Gzam and others examine a suite of beach ridges along the Tunisian coast to better understand their genesis and dynamics. The results, which integrate petrographic and facies analysis, transverse profile surveys, and field observations, suggest that these progradational, Mid-Holocene to recent beach ridges formed during periods of relative highs in sea level. The data also reveal that the modern beach ridge is composed mainly of shell debris, whereas the Holocene succession consists of siliciclastic sand, suggesting that beach ridges were nourished from two distinctive sediment sources.




Thursday, July 3, 2014

Highlights—Deepwater Fan Break-in-Slope Breakdown

Many deep-water fans include intimately associated channels and lobes, formed by dynamic conditions of flow, sedimentation and erosion, and gradient. Fernandez et al. describe a series of large-scale experiments on non-channelized turbidity currents that illustrate the evolution and complex stratigraphy of channel-lobe systems developed updip and downdip of a break in slope. Data and analysis examined: (i) velocity and suspended sediment concentration of the flows themselves; (ii) time and spatial evolution of channel and lobe construction, and (iii) spatial trends in grain-size distribution along the deposit. The results provide a comparative picture of the gross structure of the fans, with information on their surfaces, growth dynamics, and times of activity of the incised channels and lobed features. Of particular note is the role that the break in slope played in governing channel aggradation and lobe architecture over the deposit, and ultimately controlling the dimension, geometry, and connectivity of the deposits.


Growth patterns of subaqueous depositional channellobe systems developed over a basement with a downdip break in slope:laboratory experiments by Rocio Luz Fernandez, Alessandro Cantelli, Carlos Pirmez, Octavio Sequeiros, and Gary Parker


Tuesday, July 1, 2014

A Look Back…50 Years: Tufa and Algae

Recent discoveries of hydrocarbons in pre-salt carbonate deposits of the South Atlantic have generated significant interest in microbial influences on carbonate mineral precipitation. Fifty years ago, Scholland Taft broke with conventional wisdom (“…that inorganic processes of tufa deposition at Mono Lake are so dominant that tufa formation by algae is not significant”) and suggested the important role of algae on mineralization. The careful field, slab, and petrographic observations were interpreted to reflect the situation in which “algae initiate much of the precipitation and thereby fashion the calcitic or aragonitic framework of lithoid tufa.”


Algae, contributors to the formation of calcareous tufa, Mono Lake, California by David W. Scholl and William H. Taft, Journal of Sedimentary Petrology, v. 34, p. 309-319.

Tuesday, June 24, 2014

A Look Back…25 Years: Mixed Dolomite

The “dolomite problem”—how great thicknesses of dolomitized platform carbonates form—has vexed geologists for decades. In this paper, Humphrey and Quinn described the nature and distribution of dolomite from three late Pleistocene raised reef terraces in southeastern Barbados, West Indies. The data facilitated a conceptual model that was then numerically tested by computer simulation. The results revealed the how interaction of sea level fluctuations, sedimentation, rapid recurring dolomitization, and subsidence controlled generation of dolomitic successions.  


Coastal mixing zone dolomite, forward modeling, and massive dolomitization of platform-margin carbonates by John D. Humphrey and Terrence M. Quinn, Journal of Sedimentary Petrology, v. 59, p. 438-454.