Boulder, Colo., USA - A new GSA Bulletin study uses tree rings to document arroyo evolution along the lower Rio Puerco and Chaco Wash in northern New Mexico, USA. By determining burial dates in tree rings from salt cedar and willow, investigators were able to precisely date arroyo sedimentary beds 30 cm thick or greater. They then combined this data with aerial imagery, LiDAR, longitudinal profiles, and repeat surveys to reconstruct the history of these arroyos.
Arroyos are deep, oversized channels that have vertical or steeply cut walls made up of silt, clay, or sand. Because of this makeup, arroyo systems are inherently unstable, shifting at the century to millennial scale between broad floodplains and incised, high-walled channels, in which floods have a high stream power that causes more rapid erosion.
Study authors Jonathan Friedman of the U.S. Geological Survey and colleagues note that although the channels of the Rio Puerco and Chaco Wash are narrow, like an erosional gully, and uniform, like a human-made ditch, they are, in fact, natural depositional features.
Along both rivers, erosion occurred until the 1930s in association with extremely high flows. Subsequent infilling was caused by vegetation growth, channel narrowing, increased sinuosity, and vertical accumulation of sediments. Friedman and colleagues write that strongly depositional sediment transport interacted with floodplain shrubs to produce a characteristic narrow, trapezoidal channel. However, the 55-km study area along the Rio Puerco demonstrated upstream progression of arroyo widening and filling, but not of arroyo incision, channel narrowing, or floodplain vegetation development.
Friedman and colleagues conclude that the occurrence of upstream progression within large basins like the Rio Puerco makes precise synchrony across basins impossible. Arroyo wall retreat is now mostly limited to locations where meanders impinge on the arroyo wall, forming hairpin bends, for which entry to and exit from the wall are stationary.
The team notes that with the Rio Puerco's current rate of sediment deposition and long-term bedrock erosion, it would take the arroyo 310 years to completely fill in.
Processes of arroyo filling in northern New Mexico, USA
Jonathan M. Friedman et al., U.S. Geological Survey, Fort Collins, Colorado, USA. Published online on 16 Sept. 2014; http://dx.
Other GSA BULLETIN articles (see below) cover such topics as
1. The Hell Creek region, Montana, and catastrophic mass extinction at the Cretaceous-Paleogene boundary;
2. The old volcanic edifice of La Gomera, Canary Islands; and
3. The Pannonian Basin and the Ukrainian Carpathians.
GSA BULLETIN articles published ahead of print are online at http://gsabulletin.
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High-resolution chronostratigraphy of the terrestrial Cretaceous-Paleogene transition and recovery interval in the Hell Creek region, Montana
Courtney J. Sprain et al., Dept. of Earth and Planetary Science, University of California, Berkeley, California 94720, USA. Published online on 16 Sept. 2014; http://dx.
Sedimentary rocks that record the catastrophic mass extinction at the Cretaceous-Paleogene boundary (KPB) are dated in this work by Courtney J. Sprain and colleagues with unprecedented precision and over a broader time interval than previous studies. These precise data provide improved clarity of terrestrial ecosystem evolution from just before the KPB through recovery, and have been used to update the timing of magnetic polarity reversals, which are benchmarks for timing of events surrounding the mass extinction. These new data show that terrestrial species' decline started well before the KPB, suggesting that the Chicxulub bolide impact may not be the sole cause of the mass extinction. Further, these data challenge the current duration for important polarity intervals and events tied to them, including Deccan Traps volcanism, a potential player in the mass extinction. Finally, these results show that terrestrial faunas recovered within about 900,000 years of the mass extinction, an unexpectedly rapid pace.
Deformation of the substratum of a large shield volcano: Triggering factor for past flank collapses in the old volcanic edifice of La Gomera, Canary Islands
Carlos Fernández et al., Departamento de Geodinámica y Paleontología, Universidad de Huelva, Campus de El Carmen, 21071-Huelva, Spain. Published online on 16 Sept. 2014; http://dx.
The early shield volcano of La Gomera (Canary Islands) was subjected to several huge flank collapses. Deformation of the volcano basement, partly related to regional tectonics, triggered the volcano destabilization and also conditioned the geometry and kinematics of the collapsed area. Spectacular deformational structures include rotation of large blocks of La Gomera basement affecting a dense swarm of mafic dikes, which developed kilometer-scale fault-bend folds with rounded hinges associated with the staircase geometry of extensional faults. Other factors that conditioned the collapse of the volcanic edifice were cone de-buttressing, massive dike intrusion, hydrothermal activity, and seismicity.
Interplay between the thermal evolution of an orogenic wedge and its retro-wedge basin: An example from the Ukrainian Carpathians
B. Andreucci et al., Dept. of Geosciences, University of Padua, Italy. Published online on 16 Sept. 2014; http://dx.
The Carpathian-Pannonian region is made up of the wide extensional Pannonian Basin surrounded by the Carpathian mountain belt. The Pannonian Basin formed in the Miocene by extension in a retro-wedge position while thrusting was active at the Carpathian front. The Ukrainian region is an ideal area to reconstruct the relationship between the Pannonian Basin and the Carpathians because of the relatively simple structural setting and the progressive but neat transition between the two domains. This study by B. Andreucci and colleagues uses low-temperature thermochronometry and vitrinite reflectance analysis to investigate the effect of the opening of the Pannonian Basin on the thermal and burial-exhumation histories of the Ukrainian Carpathians. Timing and the spatial pattern of exhumation are compatible with post-thrusting erosion enhanced by isostatic uplift. The extent of exhumation decreases toward the Pannonian Basin, characterized by a thinned crust. The two domains appear controlled by the same lithospheric processes.
Interactions between axial and transverse drainage systems in the Late Cretaceous Cordilleran foreland basin: Evidence from detrital zircons in the Straight Cliffs Formation, southern Utah, USA
Tyler S. Szwarc et al., University of Utah, Dept. of Geology and Geophysics, 115 S 1460 E, Sutton 383, Salt Lake City, Utah 84112 USA. Published online on 16 Sept. 2014; http://dx.
The evolution of drainage systems in ancient foreland basins is often difficult to decipher; in the case of the Late Cretaceous Cordilleran foreland basin of southern Utah, USA, tectonics, eustasy, and climate fluctuations were likely forces that modified the behavior of major river systems. Through the use of detrital zircon geochronology and detailed sedimentological analyses, Tyler S. Szwarc and colleagues have linked patterns in fluvial and marginal marine stratigraphic architecture with episodes of tectonic activity in the nearby Sevier fold-thrust belt and Mogollon highlands.
Late Holocene glacial advance and ice shelf growth in Barilari Bay, Graham Land, west Antarctic Peninsula
Andrew J. Christ et al., Dept. of Geosciences, Hamilton College, Clinton, New York 13323, USA. Published online on 16 Sept. 2014; http://dx.
Prior to recent rapid regional warming, tidewater glaciers and ice shelves in the Antarctic Peninsula were at advanced positions during the Little Ice Age (LIA). In Barilari Bay, a fjord in the west Antarctic Peninsula, glaciers advanced to their maximum positions since the last ice age, supporting a fjord-wide ice shelf. This reconstruction by Andrew Christ and colleagues is based upon new multibeam data and sediment cores that show fjord grounding lines (abandoned two centuries ago) that were associated with rapid accumulation of turbidite facies in the central fjord and reduced primary productivity due to increased ice cover in the outer fjord. Their study presents the first precisely defined and dated LIA glacial advance in the region. Glacial advance in Barilari Bay was broadly in sync with glacial advance events in the south east Pacific and coincides with periods of intensified cooling in the Arctic, suggesting the LIA was truly global in scope.
Neoproterozoic to early Paleozoic extensional and compressional history of East Laurentian margin sequences: The Moine Supergroup, Scottish Caledonides
Peter A. Cawood et al., Dept. of Earth Sciences, University of St. Andrews, Irvine Building, North Street, St. Andrews, Fife KY16 9AL, UK. Published online on 16 Sept. 2014; http://dx.
From the abstract: The presence of early Neoproterozoic siliciclastic sedimentation and deformation in the Moine Supergroup, Scottish Caledonides, and equivalent successions around the North Atlantic and their absence along strike in eastern North America reflect contrasting Laurentian paleogeography during the breakup of Rodinia. The North Atlantic realm occupied an external location on the margin of Laurentia, and this region acted as a locus for accumulation of detritus (Moine Supergroup and equivalents) derived from the Grenville-Sveconorwegian orogenic welt, which developed as a consequence of collisional assembly of Rodinia. Neoproterozoic orogenic activity corresponds with the inferred development of convergent plate-margin activity along the periphery of the supercontinent. In contrast, in eastern North America, which lay within the internal parts of Rodinia, sedimentation did not commence until the mid-Neoproterozoic (about 760 million years ago) during initial stages of supercontinent fragmentation. In the North Atlantic region, this time frame corresponds to a second pulse of extension represented by units such as the Dalradian Supergroup, which unconformably overlies the predeformed Moine succession.