Thursday, November 26, 2015

Highlights—The Rise and Fall of Lakes

Rapid tectonic and climate changes can markedly influence lake extent and character, with distinct expression along- and across-strike. Ghinassi et al. suggest that although the morphological along-strike variability of lacustrine coasts is relatively well understood, the nature and signature of this variability on the sedimentological and stratigraphic record of lacustrine systems is poorly documented. This study documents how lacustrine water-level oscillations are recorded in different depositional systems along strike in late Holocene shoreline deposits of Lake Hayk of Ethiopia. The results illustrate low-relief margins with fluvial-deltaic systems and high-relief margins with collvial fan deltas and stromatolitic biostromes. The data reveal possible pitfalls of classical stratigraphic approaches used to establish along-strike correlation between lacustrine sedimentary successions, important for paleoenvironmental studies, subsurface exploration and stratigraphic investigations.

Lacustrine facies in response to millennial-century scale climate changes (Lake Hayk, northern Ethiopia) by Massimiliano Ghinassi, Filippo D’Oriano, Marco Benvenuti, Mariaelena Fedi and Stanley Awramik

Wednesday, November 18, 2015

Highlights—Cratonic Carbonate Condensation

Over Phanerozoic time scales, sedimentary records from the continental interior, or craton, tend to be thinner and less stratigraphically complete than coeval deposits from the continental margin, where subsidence allows marine basins to accumulate thick sedimentary records. Here, Brady tests whether the relationship between subsidence and stratigraphic completeness holds true at the finer temporal scales during which facies and cycles accumulate. The results of a quantitative comparison of Devonian carbonate records from the craton and continental margin suggest that, over this ~5 m.y. time period, the thin cratonic record is a result of low sedimentation rates and submarine omission, rather than minimal subsidence and increased potential for subaerial exposure. This study demonstrates how a quantitative approach can lead to new insights into important (non-)depositional processes and towards improved sampling strategies when comparing sedimentary records from distinct basins.

Tuesday, November 10, 2015

Highlights—Spatial Patterns of Fluvial Sandbodies

Distinguishing allogenic and autogenic processes on the distribution and patterns of channel belts of fluvial systems is challenging because both can control avulsions across a range of time scales and because of incomplete preservation. In this paper, Flood and Hampson present a quantitative analysis of the distribution of channelized fluvial sandbodies within the widely studied Blackhawk Formation of the Wasatch Plateau, central Utah. The results indicate that: 1) spatial patterns of sandbody distribution can be attributed to avulsion of deltaic distributary channels in locations downstream of long-lived avulsion nodes in the lower part of the Blackhawk Formation, or by compensational stacking of sandbodies in the upper part of the Blackhawk Formation, and 2) tectonic subsidence rate varied markedly during deposition (c. 80-700 m/Myr), but any potential variation in its relationship to avulsion frequency had little influence on avulsion style. This study demonstrates the importance of collecting large outcrop datasets, which enable quantitative characterization of sandbody distributions and related stratigraphic architectures using spatial statistical methods.

Tuesday, August 18, 2015

A Look Back…50 Years: South Florida Sediment

A lasting paradigm in carbonate sedimentology is that “carbonates are born, not made,” a truism that distinguishes them from the siliciclastic relatives. Fifty years ago, a classic contribution by Swinchatt emphasized the impact of their distinct origins and characteristics of early alteration on composition and texture of sediment of the South Florida reef tract. The results illustrated the complex influences of seagrass, physical process, biological breakdown, and how they vary across this shelf margin. Swinchatt suggested that “interrelationships between various rates of production and breakdown may be extremely complex and their effect on the sediment difficult to evaluate…and that further investigation is needed.”

Thursday, August 6, 2015

Highlights—Crevasse Subdeltas: Small but Important

The geomorphology, stratal architecture, and sedimentologic attributes of a range of deltas have been interpreted in the context of fluvial, tidal, and wave processes and sea-level change. Most studies focus on delta-front regions, whereas finer (and typically less well exposed) delta plain deposits have received less attention. Here, Gugliotta et al. focus on defining and describing tide-influenced crevasse subdelta deposits, an important component of lower delta plain stratigraphy of a river-dominated delta in the Lajas Formation of the Neuquén Basin of Argentina. The study emphasizes the importance of understanding the relative importance of tide and river processes in facies distribution and architecture, and the applications to characterization of interdistributary deposits. The data suggest that some “tidal” deposits interpreted from the rock record may instead be river-dominated, tide-influenced crevasse subdeltas, and that this distinction has important implications for understanding paleogeography and predicting reservoir geometries.

Stratigraphic record of river-dominated crevasse subdeltas with tidal influence (Lajas Formation, Argentina) by Marcello Gugliotta, Stephen S. Flint, David M. Hodgson, and Gonzalo D. Veiga