Public Release: 

Oct. GSA Bulletin media highlights

Geological Society of America

Boulder, Colo. - The October issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes a number of potentially newsworthy items. Topics of particular interest include paleogeographic research suggesting that North America and Australia were once neighbors, and new insights into earthquakes and tsunamis in Cascadia over the past 6700 years.

Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GSA BULLETIN in stories published. Contact Ann Cairns for copies of articles and for additional information or assistance.

"Canons" revisited and reviewed: Lester King's views of landscape evolution considered 50 years later
C.R. Twidale, Department of Geology and Geophysics, School of Earth and Environmental Sciences, University of Adelaide, Adelaide, South Australia 5005, Australia. Pages 1155-1172.

Keywords: L.C. King, models, landscape development, epigene forms, slope form, slope behavior, land surfaces, megageomorphology, etch forms, exhumed forms, paleosurfaces, survival, climatic geomorphology, catastrophism.

Why do landscapes vary in space and time? Fifty years ago Lester King, the distinguished South African geologist, attempted to summarize his views in a series of laws or general statements. King's "Canons" are reviewed and evaluated in light of subsequent developments. For various reasons they did not have the impact that might have been expected. Not surprisingly, for example, many of his ideas were left behind by technological advances and conceptual appreciations and innovations. Yet several of King's ideas are still useful in the understanding of landscape, and his efforts are deserving of praise. As it seems unjust to criticize without offering alternatives, alternative general statements concerning landscape evolution are suggested.

Pacific-North America plate motion and opening of the Upper Delfín basin, northern Gulf of California, Mexico
Michael Oskin, Institute for Crustal Studies, University of California, Santa Barbara, California 93106-1100, USA, and Joann Stock, California Institute of Technology, Pasadena, California 91125, USA. Pages 1173-1190.

Keywords: Gulf of California, Mexico, rifting, ignimbrite, plate motion, palinspastic restoration.

Knowing the amount and timing of continental rifting in the northern Gulf of California is important for understanding the history of the southern San Andreas fault, as well as developing models of how continents break apart. Ash flows deposited by volcanic eruptions on both sides of the Gulf can be matched like puzzle pieces to determine when and how Baja California was ripped from mainland Mexico. This study matched eight different volcanic ash flows between the Puertecitos Volcanic Province in northeastern Baja California to Isla Tiburon and adjacent west Sonora. Isla Tiburon is an island ecologic preserve near Bahia Kino, Sonora, and permission to work there was granted by the Seri Indians of Sonora.

The geologic constraints provided by the ash flows show 255 km of offset between units ranging from 12.6 million to 6.1 million years in age. With an additional 41 km of stretching accommodated in the Gulf's continental margins, the total amount of opening between the Pacific plate (Baja California) and the North America plate (Sonora) is 296 km, plus or minus 17 km.

The amount and timing of opening means the Pacific-North America plate boundary localized into the Gulf ocean basin ca. 6 million years ago and most activity on the southern San Andreas fault system occurred after this time. The amount of rifting also indicates Earth's crust stretched at least 1000% across the Gulf of California. The upper continental crust most likely ruptured near the present-day coastlines, and this rupture likely coincided with eruption of the seven 6.1 to 6.3 million-year-old volcanic ash flows now stranded on opposite sides of the northern Gulf of California.

Provenance of the Mesoproterozoic (1.45 Ga) Belt basin (western North America): Another piece in the pre-Rodinia paleogeographic puzzle
Gerald M. Ross , Natural Resources Canada, Geological Survey of Canada, 3303 33rd Street North West, Calgary Alberta T2L 2A7, Canada, and Mike Villeneuve, Natural Resources Canada, Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada. Pages 1191-1217.

Keywords: Precambrian, geochronology, provenance, Belt basin, North America.

One of the goals of earth sciences has been to determine the different paleogeographic states of the world as a function of the movement of tectonic plates through geologic history. We have seen many different paleogeographic pictures through time, most of which are quite different than today. One of the most vexing problems in earth sciences is determining former connections between plates in old rocks such as the Precambrian (older than 540 Ma). This paper examines tiny sand grains within the Belt basin, a Precambrian sedimentary basin preserved in the northwest U.S. (Glacier National Park) and adjacent Canada, that existed along this part of western North America nearly 1.4 Ga. Scientists at the Geological Survey of Canada used specialized equipment (the SHRIMP II [sensitive high-resolution ion microprobe]) to analyze individual grains of zircon contained within ancient river sands in this basin to determine which continent was the source of the sediment. Based on our results, a compelling case can be made that Australia was adjacent to western North America at this time and rivers that originated in mountainous regions of central and southern Australia fed the Belt basin. This former connection between Australia and North America was changed several hundred million years later when Australia rifted away during the birth of the nascent Pacific Ocean. One could now imagine standing in Glacier National Park 1.4 billion years ago and looking across a large lake or arm of the ocean and seeing Alice Springs, Australia on the other side.

Basin-scale dolomite cementation of shoreface sandstones in response to sea-level fall
Kevin G. Taylor, Department of Environmental and Geographical Sciences, Manchester Metropolitan University, Chester Street, Manchester M1 5GD, UK, and Rob L. Gawthorpe, Department of Earth Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK. Pages 1218-1229.

Keywords: dolomite, Cretaceous, Utah, sequence stratigraphy, sandstone.

An understanding of the processes controlling the large-scale distribution of carbonate mineral cements in sedimentary successions is key for modeling basin development and predicting the properties of subsurface petroleum reservoirs. This study documents the genetic link between the formation of dolomite cements over distances of over 20 km and sea-level fall in Cretaceous rocks of the Western Interior Seaway in Utah. The recognition that such large-scale diagenetic processes can be linked to sea-level change significantly improves our understanding of the characteristics of ancient sedimentary successions. In addition, this study also documents that dolomite cementation in sandstones can be an important process, one that in the past has been considered only of importance in limestones.

Early-Middle Miocene paleodrainage and tectonics in the Pakistan Himalaya
Yani Najman, Department of Geology and Geophysics, Edinburgh University, Kings Buildings, West Mains Road, Edinburgh EH9 3JW, UK, et al. Pages 1265-1277.

Keywords: Himalaya, Indus River, detrital minerals, exhumation, foreland basin, Nanga Parbat.

The initiation and routing of the Indus River through time is controversial and of importance in its interpreted role in coupled tectonic-surface process models of the exhumation of the Himalayas. This paper documents changes in sedimentology and provenance in the foreland basin sedimentary record from which it is interpreted that the palaeo-Indus first followed its modern course cutting south through the Himalayas and into the foreland basin 18 million years ago.

Great Cascadia Earthquakes and Tsunamis of the past 6700 Years, Coquille River Estuary, Southern Coastal Oregon
Robert C. Witter, William Lettis & Associates, Incorporated, 1777 Botelho Drive, Suite 262, Walnut Creek, California 94596, USA, et al. Pages 1289-1306.

Keywords: Cascadia subduction zone, paleoseismology, plate boundaries, sea-level changes, subsidence, tsunamis.

Cascadia subduction zone earthquakes dropped tidal marshes and low-lying forests to tidal flat elevations twelve times in the past 6,700 years at the Coquille River estuary in southwestern Oregon. Buried tidal marsh and forest soils discovered along the Coquille River provide evidence for sudden earthquake-triggered subsidence of the coast and coincident flooding of the estuary. The most recent earthquake occurred in A.D. 1700 and ruptured over 900 km of the plate interface. Ecological information from buried marsh soils indicates that the former marshes subsided to as much as 1.2 to 3.0 m below their original elevations. Sand deposits overlying the buried soils suggest that earthquake-induced tsunamis traveled 10 km up the estuary. The Cascadia earthquakes documented by this study recur on average every 570 to 590 years. Intervals between earthquakes varied from a few hundred years to over 1,000. Comparisons of the Coquille record to earthquake histories from adjacent sites in Oregon, southwestern Washington, and northwestern California suggest that at least two earthquakes in the past 4,000 years did not rupture the entire length of the subduction zone. This result implies that some earthquakes on the Cascadia subduction zone break 200 to 500 km segments of the plate interface, whereas other earthquakes break the entire length of the plate interface as in A.D. 1700.


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