Thursday, May 28, 2015

Highlights—Thriving Devonian microbialite reefs

The abundance of reefs and the biota that they include vary through geologic time, punctuated by periods of rapid change. To understand the dynamics of such change, Matysik et al. document the structure, dimensions, and spatial patterns of microbial-dominated patch reefs in mixed carbonate-siliciclastic environments on the northern margin of Gondwana during the Early Devonian “crisis” in metazoan reef development. Despite different composition, these reefs show close similarities in shape and spatial arrangement with present-day coral patch reefs, suggesting comparable controls. Additionally, the strata show a close relation between metazoans and microbialites in reefs and reef-associated strata, illustrating that microbialites can compete successfully, even in the presence of metazoans, given elevated nutrient supply and elevated temperatures.

Friday, May 22, 2015

Highlights—Why is the Holder tight? The impact of early diagenesis

Diagenesis represents the progressive alteration of sediment and rock; it is never simple. To examine diagenesis, many studies of carbonate successions focus on detailed petrographic study and bulk geochemical analyses. To test the hypothesis that multiple episodes of early diagenesis (subaerial exposure) are recorded as multi-phase calcite cements, Wasson and Lohmann examine petrographic and geochemical character of the Holder Formation (Pennsylvanian, New Mexico, USA). This study integrates field observations of the phylloid-algal and microbial mounds with microsampled geochemical data from some of the key features, and clarifies the detailed diagenetic and developmental history of the unit. The results illustrate that most primary and secondary porosity of the units was occluded within the first 500 m of burial, by Early Permian time, and highlight how early diagenesis can markedly impact carbonate strata. [Ed. Note: the Osmonds knew this in 1972.)

Thursday, May 14, 2015

Highlights—Prograding Wave-Dominated Deltas

Upon transport to the ocean, sediment can be transported further by wave-induced longshore sediment transport in delta–shoreface depositional systems. Nonetheless, the nature of relations between sediment supply and wave reworking is poorly understood, yet has implications regarding shoreline and stratigraphic evolution. Using a numerical model of shoreline dynamics, Li et al. quantify the relation between wave-induced longshore sediment transport and shoreline orientation under conditions of steady sea level, and apply the insights to a case study of the Po delta-shoreface system. The results reveal that a decrease in delta progradation rate can in part be considered as an autogenic response to steady wave conditions offshore. They conclude by suggesting that wave-induced longshore sediment transport can markedly impact deltaic and adjacent shoreface shoreline progradation rates, and as such, has sequence stratigraphic implications as well.

The impact of wave-induced longshore transport on a delta–shoreface system by Liang Li, Dirk-Jan R. Walstra, and Joep E.A. Storms

Wednesday, May 13, 2015

Highlights—The Power of Two: Bayhead Deltas

The stratigraphic record represents the net product of a combination of autogenic and allogenic processes, and deciphering the relative roles of each has proven challenging. Here, Simms and Rodriguez examine the influence of tributary junctions on the rate of shoreline progradation of bayhead deltas during sea-level fall. Using a simple numerical model that incorporates downstream changes in valley geomorphology under conditions of constant rate of sea-level fall and sediment supply, the data reveal that as bayhead deltas merge, the estuarine shoreline migrates seawards at increased rates. The results provide novel insights into the architecture of strata deposited during relative falls in sea level.