Tuesday, October 22, 2013

Highlights—Getting to the Permian Source

Many terrestrial deposits record paleoclimate information. In Lower to mid-Permian deposits of the Midcontinent (USA), for example, a marked and long-recognized aridification has been interpreted based on a change more humid facies (e.g., coal, organic shale) of Pennsylvanian strata to widespread redbeds, semi-arid to seasonal paleosols (Calcisols, Vertisols), and evaporites by the mid-Permian. However, the provenance, transport and depositional processes of the voluminous Permian redbeds of the Midcontinent remain largely undefined. This paper by Giles et al. suggests that the facies of the Artinskian (Permian) Wellington Formation in Oklahoma record deposition in ephemeral to perennial lakes during a time of increasing aridity and seasonality, the latter indicated by abundant mudcracks, vertic-type paleosols, conchostracans, and lungfish burrows. The fine and uniform grain size and the geochemistry of the siliciclastic component suggest far-travelled and likely eolian transport that ultimately accumulated in both subaqueous and subaerial environments. Provenance analysis indicates the siliciclastic component was sourced primarily from the southeastern Ouachita–Appalachian orogen and the Ancestral Rocky Mountains or derivative sediment. The results provide additional constraints on atmospheric circulation in this area during late Paleozoic climatic transition.

Lakes, loess, and paleosols in the Permian Wellington Formation of Oklahoma, U.S.A.: implications for paleoclimate and paleogeography of the midcontinent by Jessica M. Giles, Michael J. Soreghan, Kathleen C. Benison, Gerilyn S. Soreghan, and Stephen T. Hasiotis

Thursday, October 17, 2013

Highlights—Permian Dust in the Wind

Studies of Permian strata of west Texas–New Mexico have illuminated many classic stratigraphic concepts, including the notion of lowstand siliciclastic bypass. Although a final, lowstand origin for the basinal siliciclastics in this area is evident, the provenance of the voluminous deposits has been widely debated, with most studies suggesting the Ancestral Rocky Mountains. Soreghan and Soreghan refute this interpretation. Results, including provenance spectra, are most consistent with sources in the Ouachita system and terranes south of the Ouachita system, with contributions from as far away as the Appalachian region.  These complex transport pathways are interpreted to have involved fluvial transport, ultimate eolian delivery, and atmospheric circulation that exhibited both zonal and monsoonal components. This paper represents the first contribution of detrital zircon approaches to this world-class system, and contributes to paleotectonic and paleoclimatic interpretations for western equatorial Pangaea. 

Tuesday, October 8, 2013

Highlights—Falling-Stage Models

Conventional sequence stratigraphic models distinguish between "forced" and "unforced" regressive strata through the character of topsets (no aggradation during forced regression) and style of shoreline trajectory (descending in forced regressive strata, flat to rising in unforced regressive strata). However, because present models contain implicit assumptions about sediment supply and the response of coastal plain and fluvial depo-systems to relative falls and rises in sea level, these two scenarios may be an over simplification of a more complex reality. To explore these dynamics, Prince and Burgess investigate how topset aggradation might develop during relative fall in sea level using a simple diffusional stratigraphic forward model. Although  not always the case,  these falling stage models demonstrate that occurrence of falling-stage topset aggradation in ancient strata has important implications for the accurate identification of systems tracts, for reconstruction of relative sea-level curves and for prediction of sediment bypass into deep-marine settings.

Thursday, October 3, 2013

A Look Back…10 Years: Complexity, Completeness, and Carbonates

Hiatal surfaces in carbonate strata are ubiquitous, based on long-term accumulation rates that are orders of magnitude slower than instantaneous rates of sediment production and accumulation. Ten years ago, Burgess and Wright used a numerical model of peritidal carbonate systems to explore the lateral extent, thickness, and completeness of facies. The results of these experiments reveal how variable processes of production, transport, erosion, and accumulation create complex facies patterns, although complexity can be modified by feedbacks within the depositional system.  Burgess and Wright concluded that preserved strata represent punctuated deposition, driven by interactions between stochastic and deterministic processes, and thus the stratigraphic record is more complex than suggested by conventional sequence stratigraphic models. 

Numerical Forward Modeling of Carbonate Platform Dynamics: AnEvaluation of Complexity and Completeness in Carbonate Strata, by Peter M. Burgess and V. Paul Wright, Journal of Sedimentary Research, v. 73, p. 637-652.