Tuesday, December 31, 2013

Highlights—Waterlogged Messinian

Beyond unsupported assertions in H.G. Wells “The Outline of History,” geologists have long argued that the Mediterranean Basin had completely dried. One important event, the Messinian salinity crisis, had global impacts on biota, oceanography, and perhaps climate.  In this study, Lugli et al. review the sedimentology and stratigraphy of cores that penetrate the evaporite fills of marginal canyons from Miocene (Messinian) strata in the Levant Basin, onshore Israel. The results reveal an absence of primary in situ evaporites; instead, the cores include only clastic sulfate facies, deposited by subaqueous gravity flows sourced from dismantled selenite rocks originally located eastward and updip of the canyons. This association and the presence of the evaporite layers at different elevations along the canyons are interpreted to be the result of subaqueous mass-wasting phenomena, and that evaporites provide no evidence for marked sea level drop during the salinity crisis. On the contrary, the widespread presence of clastic evaporites is interpreted to suggest that a broad water body persisted through the acme of the salinity crisis.

Evidenceof clastic evaporites in the canyons of the Levant Basin (Israel): Implicationsfor the Messinian salinity crisis by Stefano Lugli, Rocco Gennari, Zohar Gvirtzman, Vinicio Manzi, Marco Roveri, and B. Charlotte Schreiber

Thursday, December 19, 2013

A Look Back…50 years: Hypnotizing, Mesmerizing Percussion

In December 19631, as Frankie Valli2 and the Four Seasons enjoyed a very special night, Donald Campbell excited JSP readers on percussion. Campbell’s study examined percussion marks, surficial crescentric cracks, on quartz grains from Cambrian and Cretaceous sandstones of central Texas. His results suggested that percussion marks are diagnostic of eolian transport. Unlike that special night (that ended much too soon), however, percussion marks are durable and can persist for several episodes of sediment recycling. As such, they alone are not indicative of deposition in an eolian environment.

PercussionMarks on Quartz Grains: Donald H. Campbell, Journal of Sedimentary Petrology, v. 33, p. 855-859.

Footnotes by a loopy PaperClips editor:
1 The song, describing a romantic encounter, actually came out in 1976, when it hit #1 in both the US and UK charts. A remix hit #14 in the US Billboard Hot 100 chart in 1994.  But hopefully PaperClips readers understand and appreciate the pop culture-savvy reference.

2 Editor’s note: Valli was actually not the lead singer on this track, drummer Gerry Polci provided lead vocals. But by all accounts, Frankie was the leader of the group.

Monday, December 16, 2013

Highlights—Dolomitization sans Evaporites

Many dolomitized platform carbonate successions are associated with evaporites, and many studies link dolomite to reflux associated with hypersaline brines. In this paper, Rott and Qing describe petrographic observations and geochemical data to explore the genesis of pervasive matrix-replacive dolomite in a succession not associated with evaporites, the Mississippian Alida Beds in the Williston Basin of Canada. Petrographic observations and δ13C compositions suggest an initial dolomitizing event driven by slightly evaporated seawater (but not enough for gypsum saturation), although depleted δ18O values, elevated 87Sr/86Sr ratios, and correlation of dolomite crystal sizes and δ18O values are interpreted to represent variable recrystallization of this early dolomite.  This proposed conceptual model may explain the occurrence of pervasive, matrix-replacive dolomite in the absence of depositional evaporites, and may be appropriate for other regions and hydrocarbon pools.

Thursday, December 12, 2013

Highlights—The world in a grain of sand…

Study of a suite of individual grains can provide incredible information on depositional systems and tectonic history. In this study, Vozárová et al. describe U-Pb (SHRIMP) detrital zircon ages from metasandstones of six stratigraphic levels of the low-grade early Paleozoic Northern Gemeric basement and its Carboniferous-Permian cover of the western Carpathians, Central Europe. The results reveal: a) a source from the Cadomian belt aged at between 550-700 Ma and from West African craton provenances, of ca. 1.8-2.7 Ga, b) recycling of Precambrian detrital zircon populations into the Carboniferous-Permian sedimentary basin fill, c) Variscan (355 Ma) peak metamorphic deformation and closure of the early Paleozoic basin, and d)  extensional processes and high exhumation and erosion rates of the Variscan collision belt. These dating results are interpreted to uniquely record the complex sedimentary, metamorphic, and tectonic evolution of this area.

Pb ages of detrital zircons in relation to geodynamic evolution: Paleozoic of the Northern Gemericum (Western Carpathians, Slovakia) by Anna Vozárová, Dušan Laurinc, Katarína Šarinová, Alexander Larionov, Sergey Presnyakov, Nickolay Rodionov, and Ilya Paderin

Thursday, November 14, 2013

A Look Back…5 Years: Valleys…Only in the Finales?

Sequence stratigraphy aims to subdivide the sedimentary record into genetically related units separated by unconformities that represent pronounced temporal gaps. As many unconformities are erosional surfaces, incised valleys might be considered their prototypical expression. Five years ago, Strong and Paola utilized an experimental basin to examine the relation between morphodynamic and stratigraphic expressions of relative changes in sea level on a simulated fluvial-deltaic system. The results revealed complex interactions of erosion and deposition within this system that continuously modified the shape of incised valleys. The results suggested that these dynamics create “stratigraphic” valleys that never had physiographic expression in the fluvial landscape. 

Valleys that Never Were: Time Surfaces VersusStratigraphic Surfaces, by Nikki Strong and Chris Paola, Journal of Sedimentary Research, v. 78, p. 579-593.