Tuesday, October 23, 2012

Highlights – Microbialites and Morphology

Understanding how microbialites record biological processes is essential for interpreting ancient microbial ecology and evolution.  To explore these processes, Harwood and Sumner evaluate how closely microbialite microstructures reflect the morphology and organization of the microbial community as opposed to other environmental or diagenetic processes in stromatolites and thrombolites in the Neoproterozoic Beck Spring Dolomite.  Here, these units preserve clotted and laminated microfabrics with variable preservation of microbial growth structures. This study demonstrates that diverse microfabrics in Beck Spring microbialites reflect both the morphology and organization of microbial communities as well as secondary degradational processes, and provides criteria for distinguishing these origins. Applying these criteria to ancient microbialites will facilitate evaluation of whether preserved diverse microstructures reflect variable preservation or an ecologically complex microbial community.

Wednesday, October 17, 2012

Highlights – Deltas and Sea Level

Sequence stratigraphic characterization represents an important means to subdivide complex stratigraphic successions, but the means to do so and the genetic implications continue to be debated.  In this paper, Zhu et al. describe sequence stratigraphic and chronometric analysis of extensively exposed outcrops of the Ferron Notom delta complex in south-central Utah, interpreted in the context of shoreline trajectory and accommodation-succession models.  A local accommodation curve of the deltaic complex is interpreted to indicate high-frequency and high-amplitude sea-level changes, and reflecting glacio-eustasy.  The paper implicates high-frequency and high-amplitude eustatic sea-level change as a control, driven by waxing or waning of small- to medium-sized ice sheets during episodic and ephemeral glaciations in the Antarctic region during the Late Cretaceous.

Milankovitch-scale sequence stratigraphy and stepped forced regressions of the Turonian Ferron Notom Deltaic Complex, south-central Utah, U.S.A. by Yijie Zhu, Janok P. Bhattacharya, Weiguo Li, Thomas J. Lapen, Brian R. Jicha, and Brad S. Singer

Tuesday, October 16, 2012

Highlights – Proglacial Processes

Past and present proglacial sedimentary systems record valuable information about critical phases of the Earth’s climatic history.  Although glacial outburst-related sedimentation has been identified in proglacial outwash systems, the behavior of glacial outburst flows when entering the sea are poorly understood. Here, Girard et al. document an Upper Ordovician proglacial sand-rich delta-front succession preserved in southwestern Libya, exploring how the proglacial delta front responded to sediment and meltwater discharge inputs of varying magnitude and frequency. The observations and interpretations, consistent with Pleistocene analogs, should be applicable to other paleoglaciations, and provide conceptual model for studies on management and vulnerability of groundwater and the development of glaciogenic hydrocarbon reservoirs.

Monday, October 15, 2012

Highlights – Palustrine Oolite – Rocks, but Hold the Roll

Marine ooids commonly are interpreted in the context of hydrodynamic processes, but palustrine ooids are less well understood.  Miller and James explore how palustrine ooids are produced in situ and without the necessity of grain movement.  Samples from the Nullarbor Plain in Australia reveal a suite of microbially produced structures within degraded minimicrite cortical laminae.  These ooids are interpreted to be controlled seasonal wet-dry alternations in the soil which, coupled with microbially mediated mineral precipitation, led to formation of laminae (and ooids) in place.

Monday, October 8, 2012

A Look Back...5 years – Seismic Geomorphology

Recognition, characterization, and analysis of paleo-geomorphic patterns in seismic data is a relatively new field.  In this contribution, Wood describes an example of how integrated seismic data, core and log information can be utilized to characterize and predict the nature and architecture of subsurface reservoirs.  This study focused on fluvial, deltaic, and shallow marine systems in the northern Gulf of Mexico, and described how morphometric analysis – constrained by log and core data, as well as experimental and modern analogs – can provide unique predictive insights into spatial variability in reservoir distribution and quality.   These quantitative analyses suggested some of the valuable information that can be provided by seismic data, beyond that offered by standard sequence analyses.

A Look Back...10 years – Self-Organized Tidal Flats

Although stratigraphers have long recognized that carbonate facies bodies are finite entities, characterizing their dimensions and interpreting the controlling parameters has remained a fundamental challenge.  In this paper, Rankey described a quantitative analysis of spatial patterns of subfacies size, abundance, and distribution using remote sensing data from the carbonate tidal flats of Andros Island in the Bahamas.  The results of the study revealed systematic trends in size-frequency distributions and of gaps between similar subfacies, patterns interpreted to represent power-law distributions.  These results were interpreted to be “inconsistent with models suggesting that tidal flats include a migrating complex of randomly distributed, randomly sized subenvironments,” but instead that the tidal flats are shaped by feedbacks and self-organization.

A Look Back...25 Years – Ooids and Environment

Ooids are common in many stratigraphic successions, and have provided insights into paleo-oceanographic, paleo-climatic, and paleo-depositional trends.  Here, Chow and James document the mineralogy, cortical fabrics, and diagenesis of ooids in a Middle and Upper Cambrian succession from Newfoundland.  The data from this succession revealed differences in the character of ooids in subtidal and intertidal environments, although the variability and preservation also were influenced by burial diagenesis.  These results illustrating facies-specific characteristics were interpreted to reflect the primacy of local environmental conditions (turbulence and topography) on the character of ooids in the Cambrian, distinct from more global controls on ooid type and distribution today.

Thursday, October 4, 2012

Highlights – Quartz Sand Cementation

Cementation in sandstone is spatially variable.  McBride documents cementation patterns in the Eureka Sandstone, a classic early Paleozoic quartz arenite sand sheet that was deposited across much of the western United States, and investigates why in some places it is friable and others is referred to as a “quartzite,” even in the same outcrop.  The results of the study illustrate how heterogeneous early quartz cementation leads to heterogeneous compaction; the culprit is interpreted to be related to the distribution of an authigenic illite coating on some grains that acted to retard quartz cementation.  The data indicate how cementation by quartz is not always a homogeneous process and quartzite and friable sandstones can exist in close proximity.

Tuesday, October 2, 2012

Highlights – Burrows and Early Dolomite

Evaluating the controls on early dolomite formation in ancient rocks is challenging due to a lack of reliable proxies that can be used to infer microscale environmental conditions of dolomitization.  Corlett and Jones examine sedimentological and geochemical differences between dolomite- and calcite-filled burrows in Devonian rocks from the Lonely Bay Formation, in the Northwest Territories, Canada.  An integration of various geochemical and sedimentological analyses explains why dolomite formed in some of these burrows whereas others are filled with calcite. The results of this study highlight the role of sulphate-reducing bacteria, oxygenation, and different types of organic material in early dolomite precipitation.

Monday, October 1, 2012

Highlights – Deltas and Deliverance

Most geologists would agree that shelf-edge delta processes are related to deep-water sand delivery, but the details of this linkage are somewhat uncertain.  Dixon et al. utilize a dataset of 29 examples of linked shelf, shelf-edge and deep-water strata and postulate that the depositional processes in operation at shelf-edge deltas (the 'process regime' of the delta) is intimately linked with the style of sediment transport on the associated slope and basin floor. The understanding of this link between delta process regime and deep-water stratigraphy provides insights into sedimentary source to sink systems and predictive conceptual models for hydrocarbon exploration.

Shelf-edge delta regime as a predictor of deep-water deposition by J.F. Dixon, R.J. Steel, and C. Olariu