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.

Wednesday, April 22, 2015

Highlights—Visualizing Dense Flows

Many deep-sea sediment deposits represent sediment that was transported by sediment-laden density flows. Yet the details of spatial and temporal fluctuations in flow structure responsible for the deposits have been poorly constrained due to technological challenges. To address this limitation, Perillo et al. present a non-invasive acoustic reflection technique to visualize the internal structure of sediment-laden flows over a wide range of sediment concentrations (5–75%), and the acoustic reflection character of resultant deposits. The results illustrate means to study such processes in the lab, and how those might be used to understand ancient analogs.

Acoustic imaging of experimental subaqueous sediment-laden flows and their deposits by Mauricio M. Perillo, Brandon Minton, Jim Buttles, and David Mohrig

Thursday, April 16, 2015

Highlights—What’s in a name?

What the heck do I call her? This question is not only asked by new parents or quarreling lovers or David Allen Coe, and it includes queries raised by geoscientists studying fine-grained sedimentary rocksshale, claystone, mudstone, mudrock, lutite, pelite, and argillite? The recent explosion of efforts to understand fine-grained rocks (motivated by recent realization of their economic significance) generates a compelling need to bring order to the discussions. To do so, this paper by Milliken proposes a tripartate compositional classification for fine-grained sedimentary rocks, those with greater than 50% particles less than 62.5 ┬Ám. The classification scheme is a function of the abundance of particles of extrabasinal origin and the preponderance of carbonate versus biogenic siliceous particles. This scheme, although simple, is highly functional in that it also separates rocks with distinct depositional settings, organic matter content, and diagenetic pathwaysand thus economic and engineering attributes.

Monday, April 13, 2015

Highlights—Testing fluvial conceptual models

Recent re-emphasis on large scale fan-shaped fluvial systems has motivated ongoing debate over the relative importance of basin-axial trunk river and distributive fluvial system (DFS) models in explaining stratigraphic patterns of ancient fluvial deposits, and how responses to changing accommodation, sediment supply (rate and caliber) and lobe switching are expressed in the stratigraphic record and deviate from idealized models. Assessment of the validity of these conceptual models at outcrop requires excellent exposures that spatially continuous in three dimensions. To explore these conceptual models, Gulliford et al. examine the hierarchy of architectural elements, stories, channel belts, channel-belt complexes, and sequences in outstanding outcrops of the Permo-Triassic Beaufort Group, South Africa. The data provide means to interpret river and floodplain processes, and  are most consistent with a distributive fluvial system model.