Tuesday, August 18, 2015

A Look Back…50 Years: South Florida Sediment

A lasting paradigm in carbonate sedimentology is that “carbonates are born, not made,” a truism that distinguishes them from the siliciclastic relatives. Fifty years ago, a classic contribution by Swinchatt emphasized the impact of their distinct origins and characteristics of early alteration on composition and texture of sediment of the South Florida reef tract. The results illustrated the complex influences of seagrass, physical process, biological breakdown, and how they vary across this shelf margin. Swinchatt suggested that “interrelationships between various rates of production and breakdown may be extremely complex and their effect on the sediment difficult to evaluate…and that further investigation is needed.”

Thursday, August 6, 2015

Highlights—Crevasse Subdeltas: Small but Important

The geomorphology, stratal architecture, and sedimentologic attributes of a range of deltas have been interpreted in the context of fluvial, tidal, and wave processes and sea-level change. Most studies focus on delta-front regions, whereas finer (and typically less well exposed) delta plain deposits have received less attention. Here, Gugliotta et al. focus on defining and describing tide-influenced crevasse subdelta deposits, an important component of lower delta plain stratigraphy of a river-dominated delta in the Lajas Formation of the Neuquén Basin of Argentina. The study emphasizes the importance of understanding the relative importance of tide and river processes in facies distribution and architecture, and the applications to characterization of interdistributary deposits. The data suggest that some “tidal” deposits interpreted from the rock record may instead be river-dominated, tide-influenced crevasse subdeltas, and that this distinction has important implications for understanding paleogeography and predicting reservoir geometries.

Stratigraphic record of river-dominated crevasse subdeltas with tidal influence (Lajas Formation, Argentina) by Marcello Gugliotta, Stephen S. Flint, David M. Hodgson, and Gonzalo D. Veiga

Tuesday, August 4, 2015

Highlights—Minibasin Dynamics

Although typically < 10 km wide, intraslope minibasins such as those in the Gulf of Mexico can include rapid subsidence, accumulate thick sediment pile, and ultimately host large hydrocarbon accumulations. Differential sediment loading on a mobile substrate (e.g., salt) can drive accommodation and stratigraphic architectures in intraslope minibasins, but sedimentologic and tectonic processes commonly are evaluated separately. This contribution by Kopriva and Kim experimentally integrates depositional and tectonic processes to investigate the relations of substrate movement and minibasin sedimentation. A silicone polymer model of a viscous mobile substrate provided the basis for a series of 2D experiments to explore the effects of variation in 1) sediment supply rate, 2) depositional style (intermittent sediment supply), and 3) the thickness of the deformable substrate on subsidence patterns and minibasin stratigraphic development. The results highlight the possible role of autogenic processes on minibasin dynamics and fill-and-spill stratigraphy.

Tuesday, July 14, 2015

Highlights—Mudstone Description Tools

Much of the sedimentary record comprises mudstone, and recent interest is driven by recognition of hydrocarbon sources, reservoirs, and seals in deposits of mudstone. Beyond this importance, they also influence groundwater dynamics and provide important perspectives on the global carbon cycle and climate and oceanography through geologic time. Yet, traditional mudstone descriptors neither capture the variability in their physical, biogenic, and chemical attributes, nor facilitate appropriate characterization and interpretation of controlling processes on their formation. Here, Lazar et al. discuss the utility of existing mudstone nomenclature, provide context for the information required to fully characterize mudstone, and recommend guidelines that facilitate consistent, repeatable, and efficient capture of key attributes to evaluate and compare fine-grained rocks. The application of their simple, integrated descriptive scheme to the Cretaceous Eagle Ford Shale demonstrates the utility of the technique for assessing and predicting rock properties.

Capturing keyattributes of fine-grained sedimentary rocks in outcrops, cores, and thinsections: nomenclature and description guidelines by O. Remus Lazar, Kevin M. Bohacs, Joe H.S. Macquaker, Juergen Schieber, and Timothy M. Demko

Tuesday, June 30, 2015

Highlights—Clinoform Geometries and Sediment

Clinoforms are fundamental sequence and seismic stratigraphic elements, reflecting information on sediment distribution in space and time to and across shelf margins. In this contribution, Gong et al. quantitatively explore relationships between clinoform-growth styles and sand- and sediment-budget partitioning across shelf margins of late Miocene-Quaternary clinoforms of the northwestern South China Sea. The results illustrate that clinoform-growth styles represent distinct stratal stacking patterns and sediment distribution. For example, downward-prograding shelf-margin clinoforms with low angles of growth trajectories (Gct) and low clinoform height (Hc) favor partitioning volumes of sediment into deep-water areas, thus are fronted by sand-rich submarine fan systems. In contrast, steeply upward aggrading shelf-margin clinoforms with high Gct and high Hc favor storage of volumes of sediment on shelf margins, and hence include mud-dominated mass-transport systems downdip of shelf breaks. Gct and Hc therefore increase linearly with sediment budget stored on shelf margins, but decrease linearly with sand- and sediment-budget partitioning into deep-water areas, provided similar sediment-supply conditions through time. The results suggest that clinoform-growth styles are thus good predictors of sand- and sediment-volume partitioning across shelf margins.

Growth styles of shelf-margin clinoforms: prediction of sand- and sediment-budget partitioning into and across the shelf by Chenglin Gong, Yingmin Wang, Ronald J. Steel, Cornel Olariu, Qiang Xu, Xiangnan Liu, and Qianhui Zhao

Monday, June 8, 2015

Highlights—Dueling pore systems from bioturbation

Bioturbation is an important post-depositional process that can alter sedimentary textures, porosity, and permeability. Here, Baniak et al. examine the influence of burrow geometry and connectivity on porosity and permeability within the Upper Jurassic Ula Formation of the Norwegian Central Graben. Spot permeametry data and numerical modeling of this shoreface succession illuminate relationships among burrow morphology, bioturbated volume, and burrow connectivity. These data provide insights for a conceptual framework for assigning bulk permeability to reservoir media, useful to better characterize hydrocarbon deliverance through bioturbated sandstone reservoirs with dual porosity and dual permeabeability systems.