Monday, April 29, 2013

Highlights—Microbes Pumping Iron(stone)

Oolitic ironstones occur quite frequently in condensed sections in the stratigraphic record and can serve as important iron resources. In this paper, Barale et al. use a Cretaceous condensed section to explore the conditions in which mixed Fe-oxide / Ca-phosphate ooids form, why they usually show an ellipsoidal shape in spite of a sub-equidimensional nucleus, and why Fe-oxide cortical layers intimately alternate with Ca-phosphate layers. Their results, which integrate cathodoluminescence, EDS microprobe, backscattered electron imaging, epifluorescence, XRD, and micro Raman spectroscopy, reveal a complex history of prolonged alterations of oxic and post-oxic conditions, interpreted to be related to alternating accumulation and winnowing, and related stop-and-start microbial activity. Collectively, the results provide new insights into the sedimentological and paleoenvironmental conditions in which oolitic ironstones form, detailing the sedimentation, shallow burial, reworking and sea-floor exposure of sediment that ultimately is composed almost exclusively of authigenic grains. 

The Role of Microbial Activity In the Generation of Lower Cretaceous Mixed Fe-Oxide–Phosphate Ooids from the Provençal Domain, French Maritime Alps by Luca Barale, Anna D'atri, and Luca Martire

Friday, April 26, 2013

Highlights—Small Carbonate Platform Evolution—No Bull!

Small reefal platforms that form in response to both tectonic and sea level changes are features ubiquitous through the Phanerozoic.  Commonly, however, these small platforms are buried and limited data are available on spatial and temporal lithofacies heterogeneity.  This paper by McNeill et al. documents the formation and timing of a ~90 km2 Pliocene-Pleistocene platform as it evolved in a tropical, convergent tectonic setting (backarc) from a siliciclastic foundation to a mixed system dominated by reefal carbonates.  The results reveal three depositional sequences defined using field relations and refined depositional ages using strontium-isotope stratigraphy, magnetostratigraphy, and coral stratigraphy. The spatial sedimentological patterns of the three sequences provide a near-modern analog that illustrates lithofacies variability that may be present in buried reefal platforms.

Depositional Sequences and Stratigraphy of the Colón Carbonate Platform: Bocas Del Toro Archipelago, Panama by Donald F. McNeill, James S. Klaus, Laura G. O'Connell, Anthony G. Coates, and William A. Morgan

Thursday, April 25, 2013

Highlights—Marine Mudstone: Neither Homogeneous nor Isotropic

Mudstone-dominated marine successions are common in the geological record, but a full understanding of their depositional processes commonly is hampered by a lack of generally accepted diagnostic criteria to distinguish between hemipelagic settling and deposition from a flowing medium.  In this paper, Dall’olio et al. integrate sedimentologic observations with analyses of the anisotropy of magnetic susceptibility (AMS) in the Miocene Marnoso Arenacea Formation of Italy to explore the utility of this technique for evaluating depositional processes.  The results reveal that maximum susceptibility axes clustered within the depositional plane along the average palaeoflow direction inferred from flute casts at the base of the nearest turbidite beds. This fabric is interpreted as largely sedimentary in origin, due to the alignment within the bedding plane of paramagnetic/ferromagnetic grains; this result contrasts with the hemipelagic settling interpretation that is commonly invoked for this unit.  Collectively, the results illustrate the value of AMS analysis in study of depositional processes.

Tuesday, April 23, 2013

Highlights—Random Rivers Stack in Stratigraphy

The two end-member models of the facies architecture of alluvial-coastal plain systems suggest that patterns are either predictable within unconformity-bounded sequences or that they are substantively overprinted by autogenic processes.   In this paper, Hampson et al. critically appraise these end-member models of fluvial stratigraphy using detailed architectural analysis of sandbodies developed at different positions within a regressive alluvial-to-coastal plain wedge in the Upper Cretaceous Blackhawk Formation of Utah. The results illustrate variations in the size and internal organization of fluvial sandbodies, and the relationship between these parameters and stratigraphic context; trends are broadly interpreted to reflect decreasing tectonic subsidence through time, with random distribution of sandbodies that represent large-scale avulsion (e.g., autogenic) processes.  These results can be applied to enhanced characterization of hydrocarbon reservoirs deposited in fluvial environments.

Monday, April 22, 2013

Highlights—Dynamic Inlets

Barrier-island landforms are common on many wave-dominated shoreline systems around the world, and ancient analogs form important petroleum reservoirs.  In this study, Seminack and Buynevich use ground-penetrating radar (GPR), sediment cores and dating to understand the facies architecture and evolution of shoreline and inlet systems of Assateague Island, MD, and to test the notion that it is related to the historical Green Run Inlet, located approximately 3 km to the south.  The authors suggest that a ‘traditional’ survey of relict inlets would have overlooked the study site due, to its lack of surficial evidence of inlet existence, and that the results illustrate the value of a multi-technique approach to study of inlet dynamics.  These results illustrate how reconstructing the facies architecture and evolutionary pathways of inlet-barrier island systems provide enhanced understanding of their geological evolution and stratigraphic framework, as well as their possible influence on coastal hazards.

Wednesday, April 10, 2013

A Look Back…5 years—Paleozoic Subtropical Carbonates

Carbonate sediments can be subdivided into two end-member realms, heterozoan and photozoan, that in many cases relate to “cool water” and “warm water” conditions respectively.  Between these end members are “subtropical” carbonates that can include a mix of both realms.  In this paper, Bensing et al. describe Lower Permian strata from Arctic Canada that include a temporally changing, sedimentologically mixed assemblage.  The sediments suggest a gradual cooling and decrease in seawater temperature, but that changes in sea level and oceanographic conditions complicated trends.  The results represented a refinement of criteria for recognizing and interpreting heterozoan carbonates in Paleozoic strata.

Monday, April 1, 2013

Highlights - The Poop on Shoreline Change

Recent studies have shown that many shorelines around the world, especially in densely populated areas, are eroding, in many cases due to the impact of humans.  In this new study, Seigguh examines the role of some non-bio-degradable materials on mitigating shoreline erosion.  By examining the transport thresholds and coefficients for a series of materials, the results illustrate that dirty diapers represent largely immobile elements, are abundant anyway, and actually provide a protective barrier against shoreline erosion.  The authors suggest that many beaches, especially those in populated regions, should be armored with diapers to prevent coastal erosion.