Wednesday, October 12, 2016

Highlights—Fans Off the Shelf

On planet Earth, most sediment moves downslope; unfortunately, how and when it does so can be complicated. To explore linkages and develop predictive conceptual models of relations between shelf and basinal sedimentary systems, Koo and others describe shelf-edge architecture and evolution of coeval basin-floor fans in Maastrichtian strata (Washakie Basin, Wyoming), a high-sediment-supply basin. The results of examining 630 wireline logs show that the progradation and aggradation style of submarine fans is strongly coupled with the behavior of correlative the shelf edge and shelf-edge deltas, a link interpreted to be driven by relative changes in sea level. This conceptual model is interpreted to be applicable to sediment supply-dominated margins, even as sea level changes, because of limited space on the shelf. In contrast, this linkage might not be appropriate in accommodation-dominated margins with plenty of storage space on the shelf.

Coupling between shelf-edge architecture and submarine-fan growth style in a supply-dominated margin by Woong Mo Koo, Cornel Olariu, Ronald J. Steel, Mariana I. Olariu, Cristian R. Carvajal, and Wonsuck Kim

Friday, October 7, 2016

Highlight—Microbial Mat Morphology—Nature or Nurture?

Microbial mat descriptions and classifications in modern peritidal systems focused on morphological descriptions, leaving the origins to be debated among beer-drinking geologists. To understand the roles of microbial communities and environmental conditions in the genesis and morphology of tidal flat microbialites, Trembath-Reichert and others applied optical microscopy and gene sequencing methods to study the microbial composition of mats from a tidal algal marsh in Caicos, B.W.I. Microscopy results are consistent with an interpretation that Cyanobacteria colonizing the initial mat surface are responsible its structure.  Although genetic data show the same relative abundances of the same Cyanobacteria in both “flat” and “biscuit” type mats, gene sequencing reveals that total diversity and community composition is significantly greater in the biscuit mat morphotype. These results suggest that mat morphology reflects time-integrated microbial response to various environmental factors, and in which the community diversity increases with time after environmental disturbance.  Morphology is not a consistent signature of component microorganisms. [Ed. Note: The importance of Genes cannot be understated.]

Gene sequencing-based analysis of microbial mat morphotypes: Caicos Platform, British West Indies by Elizabeth Trembath-Reichert, Lewis M. Ward, Sarah P. Slotznick, Steven L. Bachtel, Charles Kerans, John P. Grotzinger, and Woodward W. Fischer  

Tuesday, October 4, 2016

Highlights—The Bottom Drops Out in Mexico

Although potential central to understanding the evolution of both the southern part of North American Cordillera and the opening of the Gulf of Mexico, Early Cretaceous strata of Mexico have remained enigmatic. To better understand tectonic and stratigraphic evolution in this area, Sierra-Rojas and co-authors describe Lower Cretaceous strata of the Tentzo basin of southwestern Mexico. Integrating outcrop observations, petrography, and detrital-zircon geochronology, the study reveals a thick succession of red beds (conglomerate, sandstone, siltstone) overlain by carbonates, collectively interpreted to represent alluvial, lacustrine, fluvial, tidal, and marine environments. The thick succession, characterized by rapid accumulation (3.6 mm/yr), is interpreted to represent a backarc basin in an extensional setting on the paleo-western margin of Mexico, but that in time became part of the broad, stable platform facing the Gulf of Mexico. The results suggest that these basins in southwestern Mexico are more closely related to a Pacific volcanic arc, and are not related directly to the opening of the Gulf of Mexico, helping to constrain tectonodynamics.

Tuesday, August 16, 2016

A Look Back: 50 Years—Of Spots and Stains…

Out, damn'd spot!”  Lady Macbeth.  As we at JSR PaperClips continue our celebration of 400th anniversary of the Bard’s demise, we recall another famous stained contribution—that of Tony Dickson, who 50 years ago pointed out how staining of carbonate rocks with alizarin red-S and potassium ferricyanide can aide in differentiation of orthorhombic carbonates (such as aragonite), trigonal carbonates (such as calcite), and iron-rich calcite and dolomite. So, with clear conscience and citing little risk, we recommend re-reading this classic.

Monday, July 25, 2016

Highlights—Generous Shellfish? Yes, Oysters Share Insights!

Secret, and self-contained, and solitary as an oyster,” wrote Dickens.  But he also knew that no oyster lives independent of his environment. In this contribution, Sælen and others use this concept to attempt to use  δ18O, δ 13C, 87Sr/86Sr and elemental analysis of oyster shells to unravel paleoenvironmental settings of carbonates and mixed siliciclastic-carbonates from Miocene strata in the Lorca Basin of Spain. The raw and modeling results expand on patterns derived from contemporaneous corals and reveal novel insights into paleosalinities of marginal marine water in this basin. More generally, the results highlight the use of the well-preserved, low-Mg calcite shells of oysters to assess short-term changes in sea water salinity and temperature.

Oyster shells as recorders of short-term oscillations of salinity and temperature during deposition of coral bioherms and reefs in the Miocene Lorca Basin, SE Spain by Gunnar Sælen, Ingelin Løkling Lunde, Kristin Walderhaug Porten, Juan C. Braga, Siv Hjorth Dundas, Ulysses Silas Ninnemann, Yuval Ronen, and Michael Richard Talbot

Monday, July 18, 2016

Highlights—Islands in the Stream (with no Vegetation)

Interpretation of fluvial strata in continental basins is contingent upon recognizing stratal architectures and climate change signals from fluvial strata, and provide insights into understanding how fluvial sedimentation interacts with basin topography. These aspects are difficult to assess in the pre-Devonian fluvial rock record, because comparisons to models based on vegetated modern rivers may not be suitable. In this paper, Lowe and Arnott carefully describe the architecture of braided and ephemeral facies in the Cambro-Ordovician Potsdam Group in the Ottawa Embayment and Quebec Basin in northeastern North America. The contribution reveals pre-Devonian fluvial architectures and processes, and how they relate to global orbitally-forced climate changes in the Late Cambrian. These aspects of pre-Devonian fluvial sedimentology are important to the Joe Geologist because they provide a framework to make basin-wide time-significant stratigraphic correlations, and understand variations in stratal architectures with varying degrees of interaction with basement topography, potentially important for groundwater and oil migration, and discerning climate change during this critical period.

Tuesday, July 12, 2016

Highlights—In the Backwater of Tidal–Fluvial Interactions

As noted by Elvis and Ecclesiates, rivers flow surely to the sea. Yet, exactly what happens to rivers just before they get there is not well constrained, but has been interpreted to control avulsion nodes, and therefore, distributary channel patterns. To test scenarios of backwater hydraulics (especially water-surface drawdown) on sedimentology and morphology of lower-delta plain distributary channels, Columbera et al. describe field observations of the Cretaceous Neslen Formation (Campanian, Mesaverde Group) of the Book Cliffs in Utah. The results reveal ribbon sand bodies with architecture, lithofacies, and bounding surfaces and strata that are broadly consistent with patterns that would be expected in the region of rivers where the streambed drops below sea level.  Nonetheless, the authors conclude by noting a need for additional research on these processes in the rock record, and the appropriate revision of sequence stratigraphic models.

Tuesday, July 5, 2016

Highlights—Probertite and glauberite: the most interesting minerals you’ve never heard of….

Compared to their sulfur-rich evaporitic relatives, the origin and significance of sedimentary borates is poorly constrained, yet these deposits can form economic resources. To better understand lacustrine borates, Ortí et al. describe the sedimentology, petrographic characteristics, and stratigraphy of cores from Miocene strata of three exploratory boreholes from Turkey. The results reveal a succession rich in several scales of cyclic alternations of primary (depositional to interstitial) glauberite (Na2Ca(SO4)2) and probertite (NaCaB5O7(OH)4·3H2O) (yes, those will be on the quiz).  These minerals and their varied textures are interpreted to vary according to lake levels, chemistry, and paleogeographic setting in the paleo-saline lake. The results emphasize the diversity of hydrogeochemistry that can influence lacustrine evaporite depositional systems. 

Thursday, June 23, 2016

A Look Back…50 Years: Muddy Belize Models

A half century ago, Matthews bemoaned that fine-grained carbonate sediment had been neglected, except in the Bahamas. To rectify this situation, he examined mineralogy of carbonate mud offshore of present-day Belize, and found aragonite (as in the Bahamas), but with an “unusual abundance of” high-Mg calcite, in many cases between 30–70%. Matthews interpreted the mud to have two general sources: production on shoals followed by transport, and in situ production. He concluded, “If this investigation has served no other purpose, it has demonstrated that lime mud may have multiple origins.” 

Tuesday, May 17, 2016

Highlights—Sediment on the Slippery Slope

Unlike fish, carbonate platforms commonly drown.  After they do, pre-existing margins can impart large-scale inflections in bathymetry below the subsequent shelf break that influence sediment dispersal on the slope.  Here, Hurd etal. explore the relation between a pre-existing, relict break in slope and subsequent accumulations in Permian strata of West Texas. The data reveal that increased slopes favor bypass and channels, which in turn favor accumulation of downdip, onlapping organic-rich shale and carbonate mud-dominated mass transport complexes. These results are interpreted to have general applicability to drowned carbonate platforms. And, no fish were harmed during this study either.     

Highlights—Mixing it Up in the Neuquén Basin

High-resolution sequence stratigraphy can be challenging, but mixed carbonate-siliciclastic systems offer unique challenges, but at times unique insights as well.  In this paper, Schwartz et al. document outcrop and subsurface data from a Cretaceous proximal to distal transect of the Neuquén Basin of western Argentina, exploring controls on high-resolution stratal patterns. The results suggest transgressive carbonate hemicycles, overlain by storm- and wave-influenced siliciclastic shoreface deposits during regressions. The data suggest that autogenic or eustatic controls did not control facies patterns; rather changes in sediment supply related to more arid–more humid shifts are interpreted to control the patterns in these strata.  These results emphasize the potentially important role of climate shifts on stratigraphy, and highlights its potential use for correlation.

Friday, April 1, 2016

Highlights—Sequence Stratigraphy Review

Following up on his recent magnum opus that considered universal sequence stratigraphy, Burgess explores the depth of “what we really know” about the fundamental controls on sequence stratigraphy in his most recent contribution.  Wasting no words, with nary a flaw in style or content, he lucidly and simply describes his perspective on this question, and in doing so, illuminates gaps in understanding. His unusually succinct style carefully avoids the jargon common in sequence stratigraphy, so even beginners can grasp his points. [Ed. Note: We appreciate Dr. Burgess’ brief abstract, shorter than even many physics articles, which are notoriously curt.  Even though it is a review paper, it is the early favorite to win the Outstanding Paper Award.]

Wednesday, February 24, 2016

Highlights—Muddy Coastal Systems

Mud-dominated coastal systems play by their own rules. Many of the generalizations derived from years of study of sandy shoreline systems appear to not apply to shaley successions, or at least apply in unique ways. To better understand spatial changes and proximal-to-distal facies relationships of muddy systems, Harazim and McIlroy describe sedimentological, ichnological, and geochemical characteristics of the Lower Ordovician (Tremadocian) Beach Formation, Bell Island Group, Newfoundland. The results demonstrate that ancient fine-grained coastal systems are incompletely incorporated into sequence stratigraphic models owing to their atypical proximal-to-distal facies relationships, unique physical sedimentologic properties, and burial efficiency. The interpretations highlight the importance of 1) frequency of sediment supply events, 2) direction of mud transport, 3) diagenetic reactivity of minerals and bioavailable organic carbon, and (4) residence time of mineral grains and organic matter close to the sediment–water interface on facies architecture of this non-uniformitarian paleoenvironment, deposited prior to the evolution of soils. Face it, mud is complicated.