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, 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.

Tuesday, April 7, 2015

Highlights—Concretions: What goes down must come up!?

Septarian concretions provide potential geochemical archives of diagenetic evolution. Many previous studies have employed traditional oxygen isotope analyses as a major guide for paragenetic interpretation. However, the oxygen isotope compositions of carbonates are controlled by both temperature and fluid compositions, generating interpretive ambiguity, particularly in diagenetic systems where these controls are difficult to constrain. Here, Loyd et al. use the clumped isotope paleothermometer, an independent temperature proxy, to better characterize paragenesis of septarian concretions from multiple localities. Although originating from different locations and depositional systems, most concretion geochemical compositions conform to a common paragenesis. Particularly intriguing are signals of late-stage spar precipitation from meteoric fluids, interpreted to occur during uplift. These findings are inconsistent with progressive cementation during burial ('on the way down'), and instead suggest that septarian concretions form over relatively long time scales.

Clumped-isotope constraintson cement paragenesis in septarian concretions by Sean J. Loyd, J.A.D Dickson, James R. Boles, and Aradhna K. Tripati

Wednesday, April 1, 2015

Sequence Stratigraphy of the Universe

For decades, controls on sequence stratigraphy have been a controversial topic: eustasy, tectonics, or sediment supply?  Part of the challenge in unraveling controls relates to the method’s numerous implicit or explicit assumptions (“Geomyths”), such that the most extreme critics suggest that some assumptions have no basis in reality in the world as we know it, and thus “invalidate” sequence stratigraphy. To explore this concept, Peter Burgess reviews implications of recent progress in study of quantum vacuums, a previously overlooked aspect of the cosmos, on deriving a self-consistent model of sequence stratigraphy that is generally applicable to the universe, leaving aside Earth-bound sequence stratigraphy as a case study. Beginning with the recognition that gravitational repulsion of virtual particles and antiparticles leads to a “gravitational charge” and a quantum vacuum, Burgess proves an earlier concept—fundamental dimensionless constants may in fact be neither fundamental nor constant (he is mum on dimensionality). These notions raise important sequence stratigraphic questions—can tectonics exist in zero gravity?  How does one account for virtual sediment?  What is “eustasy” or a lowstand in the absence of water-filled ocean basins, but water instead falls into black holes?  Although his analysis of the universal spectrum of forces in the universe leads him to derive an incontrovertible solution (or, at least one that is consistent with Google), he concludes by acknowledging the notion that the contribution represents “a serious analysis of a ridiculous subject, the opposite of the usual in sequence stratigraphy.”