Tuesday, December 15, 2015

Highlights—A Story of Incision

Incisions are stratigraphically important in that they record removal of previously deposited strata, and can represent sequence boundaries. Here, Ullah et al. describe scour surfaces at the top of the Ferron Sandstone Member (Cretaceous, Mancos Shale Formation, Henry Mountains, Utah) that include individual channel stories. This study shows that confluence scours have diagnostic fill facies (single set of large steep foresets) and do not produce multi-storey sand bodies. The results, coupled with modern analog studies, suggest that the maximum depth of confluence scours by autogenic process may reach up to four to five times the average depth of the incoming channels. Consequently, allogenic incised valleys (e.g., at sequence boundaries) can be defined in ancient systems only in situations wherein the erosional relief is more than five times average channel depth, markedly deeper than the thickest fully preserved storys, which in a braided stream are likely to represent confluence scour fills. Examples of autogenic modification of allogenically formed incised valleys suggest that both allogenic forcing and autogenic feedback can act simultaneously in fluvial systems.

Thursday, December 10, 2015

Highlights—Tectonics and Toxicity Take Triassic Platform to Task

A lasting question in carbonate geology asks how isolated platforms—the sites of prolific and rapid sedimentation—can drown. To explore how platform drowning occurs, Minzoni et al. document patterns within backstepping platform margin architecture that led up to the drowning of the Yangtze Platform (Triassic, South China).  Results reveal that a combination of mechanisms—anoxic basin development and tectonic subsidence/syndepositional faulting—led to drowning of the platform by sinking into toxic bottom waters. Although such a combination of mechanisms is not commonly interpreted as cause of platform demise, the abundance of anoxic tectonically active basins in the stratigraphic record suggests that the combination may be a widespread cause of platform drowning, and patterns of architecture and evolution documented here provide criteria for recognizing tectonically induced drowning in other basins. Finally, the paper philosophically explores the tragedy of platform drowning, and the sense of loss felt at yet another perfectly good platform gone under.

Drowning of the Triassic Yangtze Platform, south China, by tectonic subsidence into toxic deep waters of an anoxic basin by Marcello Minzoni, Daniel J. Lehrmann, Erich Dezoeten, Paul Enos, Paul Montgomery, Adrian Berry, Yanjiao Qin, Yu Meiyi, Brooks B. Ellwood, and Jonathan L. Payne

Tuesday, December 1, 2015

Highlights—Setting the Bar: Unit Bars and Point Bars

Many studies of fluvial systems utilize one-dimensional profiles (measured sections) and bedding and facies architecture to predict 3D architecture and grain size change. Here, Wu et al. use extensive plan-view exposures coupled with vertical cliff exposures to document channel plan-form, channel-belt dimension, bar migration patterns (translation versus expansion), and cross-sectional facies architecture in the Cretaceous Ferron Notom Delta complex. The data reveal the presence and dominance of small-scale unit bars on point-bar deposits. Commonly under-recognized in ancient deposits, these unit bars are ubiquitous features of modern fluvial systems, and may form important parts of fluvial reservoirs.