Tuesday, August 27, 2013

Highlights—Polygenetic Paleosols and Paleoclimate

Paleosols represent atmosphere-influenced sediment alteration during periods of landscape stability. To better understand paleosols and paleoclimate of the late Paleozoic ice age, two papers (Rosenau et al. and  Rosenau et al.) characterize the macro-morphology, micro-morphology, mineralogy, isotopes, and stratigraphic and lateral distribution of paleosols in Pennsylvanian (Atokan-Virgilian) mixed marine and terrestrial coal-bearing strata (“cyclothems”) in the Illinois basin. The data illustrate intra- and inter-cyclothem variability in paleosol morphology, mineralogy, and isotopes interpreted to reflect marked paleohydrologic variability driven by a combination of autogenic basin-scale controls (such as groundwater fluctuations) and allogenic regional- to global-scale controls (such as glacioeustasy and climate). These results refine understanding of the complex, polygenetic origin of these paleosols and the paleoclimate changes during deposition.

Monday, August 26, 2013

Highlights—Geochemical Correlations in Submarine Channel Sand

Unraveling stratigraphic variability represents a complex challenge in heterogeneous, fossil-poor successions. In this study of a late Miocene submarine slope–channel system in the Tabernas Basin of southern Spain, Aehnelt et al. apply X-ray fluorescence, X-ray diffraction, and SEM analyses, supplemented by light microscopy, to assess the utility of geochemical correlation techniques for correlation. The observations suggest that sandstone beds are geochemically homogenous (geochemical stratigraphy is comparable to lithostratigraphy), but in some cases, geochemical evaluation using particular enrichments, depletions, or major trends allows even more detailed correlation than simple physical correlation. The results are interpreted to suggest that geochemical stratigraphic correlations are valid over a range of scales (m to 100s of m), but that geochemical correlations need to be applied with care if utilizing a limited range of elements.

Geochemical correlation in an exhumed submarine channel complex (Tabernas Basin, SE Spain): comparison to sedimentological correlation at various length scales by Michaela Aehnelt, Richard H. Worden, Andrew C. Canham, Stephen J. Hill, David M. Hodgson, and Stephen S. Flint

Wednesday, August 21, 2013

Highlights—Tectonics, Climate, and Paleocene Paleosols

Globally warm temperatures and elevated atmospheric CO2 in the Paleocene make this epoch of particular interest for understanding greenhouse worlds; yet, terrestrial records from this interval are rare, particularly in North America. In this study, Torres and Gaines examine morphology, clay mineralogy, and bulk geochemistry of paleosols from two members in the late Paleocene Goler Formation of Southern California to investigate paleoclimatic change during deposition. The results reveal a shift in pedogenic processes concomitant with changes in provenance and sedimentation rate. These data are interpreted to suggest that paleotopographic change, driven by regional tectonic activity, was an important control on local climate and pedogenic processes.

Monday, August 19, 2013

Highlights—Alluvial Dryland Systems: Listen to Modder

Alluvial deposits have provided valuable insights into hydroclimatic variability and paleoenvironments, yet, in dryland settings, the roles of allogenic and autogenic processes are poorly constrained.  Tooth et al. investigate the controls on the genesis, sedimentary architecture and preservation potential of the alluvial succession along the modern incising Modder River, located in the tectonically stable South African interior far beyond the range of sea level influence.  The results show that whereas allogenic factors may be the primary driver of river activity in inland valleys, resistant rock barriers can exert additional complicating influences—stable barriers control the depth of cutting during climatically-driven incisional phases, but once partial or complete barrier breaching occurs, deep channel incision into bedrock will take place in reaches upstream, thus forming prominent valley-base erosion surfaces and possibly promoting the formation of incised gully networks. These findings are interpreted to provide insights that can be applied to improved interpretation of comparable dry, inland alluvial valleys preserved in the geological record.

Monday, August 12, 2013

Highlights—Paleosols: “Absence of Evidence is not Evidence of Absence”

Recent studies of paleosols have provided important insights into landscape processes and climate; yet, due to their landscape position, they may not be preserved. In this paper, Gastaldo et al. describe mudclast aggregates in anabranching deposits of the Triassic Katberg Formation of South Africa. Results reveal that the aggregates are heterogenous coarse silt to fine sand and are not found as matrix or within in situ paleosols; instead, they are associated with reworked carbonate nodules and mudchip rip-up clasts. These attributes are used to interpret the former presence of PaleoVertisols across the Early Triassic Karoo landscape. These data and recognition of mud aggregates is interpreted to reinforce the idea that mudclast aggregates are characteristic of closed depositional systems, including continental-interior basins.