December 2011 Geology highlights

November 10, 2011

Boulder, CO, USA - Articles in the December 2011 issue of Geology include the discovery of microfossils in the Tayshir Formation, Mongolia; examination of sediments in a deep crater lake on Kilimanjaro to understand past environmental change in East Africa; determination of the timing of favorable environmental conditions that would allow human migration across southern Arabia; and zircon dating results in Jack Hills, Australia, that are expected to spark debate on the true nature of the Hadean Earth.

Keywords: Kohistan Island Arc, mantle, olivine, Etendeka, Namibia, coignimbrites, West Greenland, Cretaceous, Western Interior Seaway, Ferron Sandstone, Lake Challa, Mount Kilimanjaro, palaeoclimate, modern human dispersal, Arabia, Cambrian, Mount Simon sandstone, Illinois Basin, Tayshir Formation, Mongolia. NASA EO-1 Hyperion imaging spectrometer, SAFOD, San Andreas fault, maars, Eifel Volcanic Field, magma crystallization, SHRIMP, Jack Hills, Australia, coastline morphodynamics, East Asian monsoons, paleoclimate, seafloor massive sulfide deposits, mid-oceanic ridge, neovolcanic zones, Archean, Continental Flood Basalts, Moho, fossilized burrows and trails, Kimmeridge clay, petroleum plays, hydrocarbons, UHP metamorphism, Gilbert-type deltas, wax lake

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Geological evidence and modeling of melt migration by porosity waves in the sub-arc mantle of Kohistan (Pakistan)

Pierre Bouilhol et al., Dept. of Earth Sciences, ETH and University Zurich, Sonneggstrasse 5, 8092 Zurich, Switzerland;

Intra-oceanic subduction zones are sites of intense magmatic activity that is responsible for Oceanic Island-arcs and a large part in the global geochemical cycle of Earth. The way melts migrate from the mantle wedge, where they are produced, up to the surface remained conjectural. Pierre Bouilhol and colleagues of ETH Zurich have studied mantle rocks of the fossil Kohistan Island Arc exposed in the Pakistani Himalaya. The geological and petro-geochemical characteristics of these mantle rocks provide evidence for melt migration in porosity waves. Numerical simulations support and constrain geological evidence. Porosity waves are self-propagating, fluid-filled ellipsoidal domains that collect and transport melt toward the surface, and leave behind them a porous conduit. Once formed, these conduits act as melt highways for magmas that nourish the volcanic island-arcs. Porosity waves are therefore a fundamental melt-migration mechanism in the upper parts of the sub-arc mantle wedge.

On the significance of ultra-magnesian olivines in basaltic rocks

J.K. Keiding et al., GFZ German Research Centre for Geosciences, Telegrafenberg, D-14473 Potsdam, Germany;

The Mg/Fe ratio of olivine is frequently used to constrain the temperature of melts from which it formed. The highest temperatures are suggested by ultra-magnesian olivines, those with greater than 91.5 % of the pure Mg-olivine (forsterite). These olivines are common in the so-called komatiites that formed in the early Earth when mantle temperatures were much higher than today. Occasionally, however, ultra-magnesian olivines are also observed in large igneous provinces of much younger age, and interpreted as evidence for extremely high temperatures in mantle plumes. This paper by J.K. Keiding of the German Research Centre for Geosciences and colleagues tests the relation of high temperature and ultra-magnesian olivines from the 130 million year-old Etendeka province of Namibia. They analyze minute inclusions of melt preserved within the olivine crystals. These inclusions are direct samples of melts originally in equilibrium with olivine, and they yield mantle temperatures some 150 degrees Celsius lower than those computed from the bulk-rock compositions. This finding has important ramifications for the use of olivine as a thermometer and for our ideas of the thermal structure of the mantle.

Effects of topography on pyroclastic density current runout and formation of coignimbrites

Benjamin J. Andrews, Dept. of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, 1000 Constitution Ave. NW, Washington, D.C. 20004, USA; and Michael Manga, Dept. of Earth and Planetary Science, University of California, 307 McCone Hall, Berkeley, California 94720, USA;

Pyroclastic flows and surges commonly travel over large ridges and other topographic barriers. The ability to predict how those currents of hot volcanic ash and gas interact with topography is critical to understanding explosive volcanic eruptions and making informed decisions about volcanic hazards. In this paper, Benjamin Andrews of Smithsonian Institution and Michael Manga of the University of California at Berkeley present the results of scaled laboratory experiments that examine the behavior of pyroclastic density currents. Because their experiments are conducted with warm talc powder in air, the model currents heat and expand entrained air and thus can reverse buoyancy and lift off to form buoyant ash plumes. Hot, well-mixed currents have shorter runout distances and lift off sooner than cool, poorly-mixed currents. Interestingly, comparisons of currents that encounter barriers with currents that traverse a flat floor show that there is very little change in runout distance unless the barrier is greater than 1.5 times the current thickness, suggesting a 400-m-thick pyroclastic flow could overtop a 600-m-tall ridge. Further, buoyant plumes lift off at the termini of currents on flat floors but become focused above even relatively small barriers when currents encounter obstructions. Results thus provide a means of predicting runout and liftoff behavior of natural pyroclastic density currents.

Craton formation in Late Archean subduction zones revealed by first Greenland eclogites

Sebastian Tappe et al., Institut fur Mineralogie, Westfälische Wilhelms-Universität, Corrensstrasse 24, 48149 Münster, Germany;

West Greenland comprises one of the oldest continental landmasses on Earth (up to 3800 million years old). Its formation and the reason for its long-term stability has been enigmatic -- until recently. Research by Sebastian Tappe and colleagues presents the first eclogites (high-pressure basalts containing garnet) from the continental root beneath West Greenland. These eclogites were sampled as rare fragments by violent kimberlite eruptions from around 150 km depths. Their chemical and isotopic compositions, however, show that the high-pressure eclogite material must have formed originally as part of shallow ocean floor. It is therefore suggested that this ancient oceanic crust was brought to depth via a subduction mechanism that ultimately led to closure of ocean basins and collisions between immature micro-continents and arcs. Evidence is presented that this early form of ocean floor subduction, slab melting, and subsequent continent collision occurred at around 3000 to 2500 million years ago. This sequence of events must have given rise to the formation and stability of the present-day Greenland landmass.

Foreland basin structural growth recorded in the Turonian Ferron Sandstone of the Western Interior Seaway Basin, USA

Christopher R. Fielding, Dept. of Earth & Atmospheric Sciences, University of Nebraska, 214 Bessey Hall, Lincoln, Nebraska 68588-0340, USA;

Author Christopher Fielding of the University of Nebraska provides documentation of the effects of tectonic forcing on the stacking patterns of a Cretaceous sedimentary rock unit in south-central Utah, USA. The Ferron Sandstone was formed at a time when, in this part of the USA, the Western Interior Seaway sedimentary basin was experiencing spatially and temporally variable patterns of subsidence and uplift. These patterns are evident from a 67-km-long, north-south cross-section, oriented perpendicular to the direction in which sediments were dispersed into the basin. The cross-section clearly shows an arch-like fold structure that grew during sediment accumulation, and which was planed off by a later erosion surface also within the formation. The significance of this feature is that if seen in outcrops of more limited extent, it could potentially be misinterpreted as evidence for major changes in sea-level caused by polar glaciation during the Cretaceous period. The patterns documented in this paper argue persuasively for a tectonic origin for the observed stacking patterns.

Seasonality in equatorial climate over the past 25 k.y. revealed by oxygen isotope records from Mount Kilimanjaro

Philip A. Barker et al., Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK;

A small but exceptionally deep crater lake on the flanks of Mount Kilimanjaro, East Africa's highest mountain, holds an undisturbed record of climate fluctuations for the last 25,000 years. The story locked within the sediments of Lake Challa reveals evidence of major changes from very wet to profoundly arid over this period. The data come from oxygen trapped within the silica shells of single-celled algae called diatoms. By examining the nature of the oxygen preserved in the diatoms, through the ratio of light and heavy isotopes, a long-term picture of precipitation and evaporation emerges. Variation in the oxygen isotope ratio relate to the extent and intensity of the wet and dry seasons and results from fluctuations in the monsoon climate serving the region. A comparison is made with a similar, but much shorter oxygen isotope record from the ice cap that clings to the mountain's summit. An inverse relationship is found indicating that if precipitation and evaporation control the lake oxygen isotopes as argued here, then these same water balance processes cannot also be the major control on the ice core record. These new data, and others being collected from the team of scientists working on Lake Challa, help understanding of the rate, direction, and mechanisms of environmental change in equatorial East Africa.

Humid periods in southern Arabia: Windows of opportunity for modern human dispersal

T.M. Rosenberg et al., Institute of Geological Sciences, University of Bern, 3012 Bern, Switzerland and Oeschger Centre for Climate Change Research, University of Bern, 3012 Bern, Switzerland;

Arabia is a key area for the dispersal of Homo sapiens out of Africa. Given its modern hostile environment, the question of timing is also a question of climate. Fresh water and food were crucial factors to allow Homo sapiens to populate Arabia. By dating relict lake deposits, T.M. Rosenberg of the University of Bern and colleagues indentified four periods of lake formation: during the early Holocene and centered at ~80, ~100 and ~125 thousand years ago (ka). Favorable environmental conditions during these periods allowed Homo sapiens to migrate across southern Arabia. Between ~75 and 10.5 ka, arid conditions prevailed and turned southern Arabia into a natural barrier for human dispersal. Thus, migration of Homo sapiens through the southern corridor into Asia must have taken place before 75 ka, possibly in multiple dispersals.

Evolution of quartz cementation during burial of the Cambrian Mount Simon Sandstone, Illinois Basin: In situ microanalysis of delta-18O

Anthony D. Pollington et al., WiscSIMS, Dept. of Geoscience, University of Wisconsin, Madison, Wisconsin 53706, USA;

Water flowing through sandstone will precipitate mineral overgrowths if temperature and chemical conditions are right. These mineral cements record information about temperature and water chemistry, which can be investigated with high-precision geochemical techniques. Anthony D. Pollington of the University of Wisconsin-Madison and colleagues use a secondary ion mass spectrometer to investigate oxygen isotope zonation in microtraverses of quartz overgrowths from the Cambrian Mount Simon sandstone in the Illinois Basin. Samples spanning a wide geographic and depth distribution tell a consistent story. The results indicate that quartz precipitated from water during burial and heating (~40-110 degrees Celsius). Rather than representing a single event as previously believed, these quartz cements started at near-surface conditions and grew over a period of up to 250 million years.

Putative Cryogenian ciliates from Mongolia

T. Bosak et al., Dept. of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA;

Various relatives of modern eukaryotes should have been present in the oceans between 716 and 635 million years ago (Ma), but few representatives of these organisms were preserved in the fossil record of that time. A search for microfossils in limestone strata of the 716-635 Ma Tayshir Formation, Mongolia, reveals exceptionally preserved organic-walled microscopic eukaryotes that were not described in older strata. These fossils are ~ 0.1 mm long, orange-brown, distinctly flask-shaped, with thickened collars and constricted necks and flexible walls composed of densely packed small alveoli. Morphologically, these microfossils are remarkably similar to tintinnids, modern microscopic planktonic ciliates, indicating that these plankton-grazing ciliates lived in the oceans > 100 million years before previous estimates. Therefore, ecosystems between the first and the second Neoproterozoic low-latitude glaciation (Snowball Earth) may have contained various relatives of modern eukaryotes, and some of these organisms can be recognized in the fossil record. Those eukaryotes forming recalcitrant organic or mineral-rich tests may have increased the export of organic carbon to the deep ocean, modernizing the cycling of carbon, driving an increase in atmospheric oxygen and setting the stage for the subsequent radiation of complex animals.

Constraints on determining the eruption style and composition of terrestrial lavas from space

Robert Wright et al., Hawai'i Institute of Geophysics and Planetology, University of Hawai'i at Manoa, Honolulu, Hawaii 96822, USA;

The surface temperatures of active lavas relate to cooling rates, chemistry, and eruption style. Robert Wright on the University of Hawai'i and colleagues analyzed 61 hyperspectral satellite images acquired by the National Aeronautics and Space Administration's Earth Observing-1 (EO-1) Hyperion imaging spectrometer to document the surface temperature distributions of active lavas erupted at 13 volcanoes. Images were selected to encompass the range of common lava eruption styles, specifically: lava fountains, flows, lakes, and domes. Results reveal temperature distributions for terrestrial lavas that correlate with composition (i.e., a statistically significant difference in the highest temperatures retrieved for mafic lavas and intermediate and felsic lavas) and eruption style. Wright and colleagues conclude that insights into composition and eruption style can only be gained remotely by analyzing a large spatio-temporal sample of data. This has implications for determining composition and eruption style at the Jovian moon Io, for which no in situ validation is available.

Aseismic sliding of active faults by pressure solution creep: Evidence from the San Andreas Fault Observatory at Depth

J.-P. Gratier et al., ISTerre (Institut des Sciences de la Terre) Observatoire, Université Joseph Fourier Grenoble, CNRS, BP 53, Grenoble 38041, France;

Active faults in the upper crust can either slide steadily by aseismic creep, or abruptly causing earthquakes. Creep relaxes the stress and prevents large earthquakes from occurring. Consequently, identifying the mechanisms controlling creep, and their evolution with time and depth, represents a major challenge for predicting the behavior of active faults. Based on microstructural studies of rock samples collected from California's San Andreas Fault Observatory at Depth (SAFOD), J.-P. Gratier of Univ. Joseph Fourier Grenoble and colleagues show that stress-driven mass transfer creep, such as pressure solution, can account for aseismic creep along the creeping segment of the San Andreas Fault. They used experimental data to demonstrate that the observed creep mechanism can accommodate the documented 20 mm/yr aseismic displacement rate of the San Andreas Fault creeping zone. They also show how the interaction between fracturing and sealing controls the pressure solution rate, and discussed how such a stress driven mass transfer process is localized along some segments of the fault.

Direct dating of Quaternary phreatic maar eruptions by luminescence methods

Frank Preusser et al., Dept. of Physical Geography and Quaternary Geology, Stockholm University, 10691 Stockholm, Sweden;

One important issue in natural hazards assessment is to decipher the history of volcanic fields; i.e., the frequency and nature of past eruptions. For doing so, it is necessary to securely date past eruption events, but this is difficult for maars, the second most common type of volcanoes on land. These are characterized by limited magma effusion but a high damage potential due to the sudden and explosive nature of the eruption. Frank Preusser of Stockholm University and colleagues present new approaches to date maar eruptions using luminescence methods. These methods utilize the fact that the latent luminescence signal in quartz and feldspar grains is removed by the shock wave associated with the explosive nature of maar eruptions, caused by the interaction of magma and groundwater. We tested the new approaches on known age maar deposits from the Eifel Volcanic Field, Germany, and found excellent agreement for one of the approaches. The new methodology opens up new perspectives for reconstructing the eruption history of young volcanic fields and may, in the end, contribute towards a better assessment of the volcanic hazard in potentially vulnerable areas.

Experimental evidence for crystal coarsening and fabric development during temperature cycling

Ryan D. Mills et al., Dept. of Geological Sciences, University of North Carolina, Chapel Hill, North Carolina 27599-3315, USA;

Oscillating temperature dramatically speeds up recrystallization of a magma analog consisting of ammonium-based crystals in liquid. In situ observation of crystallization indicates that crystals pulse in size during thermal cycling; over the course of hundreds of cycles larger crystals grow and smaller crystals shrink, dramatically skewing the crystal size distribution. Crystal dissolution and growth in a pulsing thermal gradient produces a pronounced fabric, with crystals aligned subparallel to the direction of heat flow. Alignment occurs via selective dissolution and growth of crystals of diverse orientations inherited from the starting material. These results have important implications for understanding how crystals grow and for interpreting texture in igneous rocks. Temperature cycling is likely common in magmatic systems and needs to be considered when analyzing chemical zoning of igneous crystals and rock textures.

Metamorphic replacement of mineral inclusions in detrital zircon from Jack Hills, Australia: Implications for the Hadean Earth

Birger Rasmussen et al., Dept. of Applied Geology, Curtin University, Kent Street, Bentley, WA 6102, Australia;

Small mineral inclusions trapped inside clastic grains of zircon more than 4 billion years old from Jack Hills, Australia, are considered to have crystallized from the same magma as the enclosing zircon and therefore to provide important clues about conditions on Earth at a time before the oldest known rocks. Results presented by Birger Rasmussen of Curtin University and colleagues suggest that most of the inclusions are much younger than the enclosing zircon grain, and formed after the sedimentary rocks were deposited. The inclusions, therefore, give no information about the source of the ancient zircons and provide no evidence for low-temperature granitic crust or ancient subduction zones. Instead, the available evidence can be explained by the existence of a Hadean protocrust composed predominantly of intermediate and mafic rocks. These observations call for a major reevaluation of evidence for the source of the oldest-known terrestrial material and have important implications for the composition of the earliest crust, the antiquity of plate tectonics and conditions on the early Earth. Their results are likely to stimulate debate on the true nature of the Hadean Earth, with implications in many fields of earth science including geochemistry, tectonics, igneous petrology, planetary geology and geobiology.

Beach and sea-cliff dynamics as a driver of long-term rocky coastline evolution and stability

Patrick W. Limber and A. Brad Murray, Division of Earth and Ocean Sciences, Duke University, Box 90227, Durham, North Carolina 27708, USA;

Rocky coastlines, with wave-battered headlands interspersed with pocket beaches, stir human imaginations and aesthetic sensibilities the way few other landscapes do. Despite their prevalence (sea cliffs or bluffs are present along nearly 75% of the world's oceanic coastlines), we know very little about how rocky coastlines evolve. Quantitative studies of large-scale rocky coastline evolution are just beginning, and this work asks several basic questions. For example, what determines the planform (map view) shape of a rocky coastline? Can it reach a steady-state configuration? How can headlands and embayments persist? Exploratory and purposefully simple analytical and numerical modeling based on the dynamics between beach sediment and sea cliff erosion show that a handful of simple physical parameters, such as sea cliff height, cliff erosion rate, and beach erosion rate could control large-scale rocky coastline evolution and morphology. Researchers studying terrestrial landscapes will recognize parallels with the evolution of mountainous and arid terrain, and those studying sediment transport and morphology will recognize rocky coastlines as a long-timescale example of how simple physical interactions can lead to interesting large-scale "morphodynamic" behaviors.

Paleosol carbonate multiple isotopologue signature of active East Asian summer monsoons during the late Miocene and Pliocene

Marina B. Suarez et al., Dept. of Geological Sciences, University of Texas, 1 UTSA Circle, San Antonio, Texas 78249, USA;

The East Asian Monsoons are the life-giving source of water for hundreds of millions of people living in China and nearby regions. In the face of climate change, it becomes important to ask how the monsoons may have differed during periods of warm climate in Earth's geologic past -- periods that may be analogous to projected scenarios of future greenhouse warming. In this study, Marina B. Suarez of the University of Texas and colleagues observe that East Asian summer monsoon rainfall is remarkably deficient in the heavy isotope of oxygen, oxygen-18, compared to summer precipitation falling elsewhere globally. Using a new isotope technique, we were able to trace this unique "signature" of the East Asian monsoons back through a period of time between seven and three million years ago when global temperatures were a few degrees warmer than present. Based on this signature, the authors infer that the monsoons originated at least 7 million years ago, and that they have been active during warm times of Earth's geological past. Whether these results can be projected into the future depends in part on other changes in the Earth System that can influence regional climate -- for example, the height of the Himalaya Mountains and the extent of the Tibetan Plateau.

The abundance of seafloor massive sulfide deposits

Mark Hannington et al., Dept. of Earth Sciences, University of Ottawa, Ottawa K1N 6N5, Canada;

Unprecedented growth in demand for metals from emerging economies is driving aggressive exploration for new sources, including in the oceans. Mining companies are already searching for copper and zinc in seafloor massive sulfide deposits. The possibility of seafloor mining has stirred debate about the use of this potential new resource and whether commercial development is worth the risk. Among the outstanding questions is how many deposits might be accessible to deep-sea mining. Estimates of the total number of seafloor massive sulfide deposits on the mid-ocean ridges range from as few as 500 to as many as 5000. Mark Hannington of the University of Ottawa and colleagues use new deposit occurrence data to estimate the amount of massive sulfide in the easily accessible neovolcanic zones of the global oceans. They conclude that the total accumulation in these areas is on the order of 600 million tonnes, containing ~30 million tonnes of copper and zinc. This is insufficient to satisfy the increase in demand for these metals and, although deep-sea mining is technically feasible, any future contribution to global metals supply will have to include long extinct deposits that may exist well beyond the neovolcanic zones.

Lower crustal flow kept Archean continental flood basalts at sea level

Nicolas Flament et al., CNRS, UMR 5276, Laboratoire de Géologie de Lyon, Ecole Normale Supérieure et Université Claude Bernard Lyon 1, Université de Lyon, 2 rue Raphael Dubois, 69622 Villeurbanne Cedex, France; and EarthByte Group, School of Geosciences, University of Sydney, NSW 2006, Australia;

Large volcanic eruptions testify of the dynamics of Earth's interior throughout its history. Such basaltic piles are particularly abundant in the Archean (between 4 and 2.5 billion years ago), suggesting important volcanic activity during that period. A representative example of such basaltic piles erupted on top of continental crust is exceptionally well preserved and well dated in the Pilbara Craton, Western Australia. Nicolas Flament and colleagues show that, despite being more than 2 km thick, this Archean basaltic pile was maintained close to sea level during its eruption that lasted less than 25 million years. In the absence of extensional tectonics, the subaqueous character of this thick basaltic calls for explanation. A series of numerical models show that hot and weak lower continental crust could flow laterally under the weight of a basaltic pile, hence impeding its emergence. They suggest that the abundance of subaqueous basaltic piles erupted over continents in the Archean indicates that the Archean continental crust was much hotter and weaker than it is at present.

The déjà vu effect: Recurrent patterns in exploitation of ecospace, establishment of the mixed layer, and distribution of matgrounds

Luis A. Buatois and M. Gabriela Mángano, Dept. of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan S7N 5E2, Canada;

After looking at the fossilized burrows and trails (trace fossils) left by animals in rocks of all ages and sedimentary environments, a research team from the University of Saskatchewan, Canada, has just published an innovative study that addresses how organisms behave when trying to colonize ancient environments. The study indicates that the initial exploitation of underutilized or empty ecological space (e.g. sediment on the marine floor or on lakes) is linked to a set of recurrent behavioral strategies developed in order to obtain food, commonly from microbial matgrounds. In the modern oceans, the sea-bottom sediment is intensely mixed by animal activity (bioturbation), preventing the establishment of matgrounds and firmgrounds. The absence of intense bioturbation at the beginning of the Paleozoic allowed the formation of widespread matgrounds and firm substrates in the sea bottom. With the advent of intense bioturbation later in the Paleozoic, a mixed layer of homogenized sediment was established. One of the most important implications of this study is the proposal that immediately after the end-Permian mass extinction that took place 251 million years ago, intense bioturbation disappeared from the sea bottom, resulting in the re-establishment of firmgrounds and matgrounds.

Can hydrocarbon source rocks be identified on seismic data?

Helge Løseth et al., Statoil ASA, Research Centre Rotvoll, NO-7005 Trondheim, Norway;

Hydrocarbon source rocks contain significant volumes of organic matter, are capable of expelling petroleum when heated, and have produced most of the world's known oil volumes. Recently, source rocks have also become recognized as unconventional economic reservoirs. In this study, Helge Loseth and colleagues present a new way of identifying, characterizing, and mapping spatial distributions and variations of thick source rocks (greater than 20 m) based on seismic data only. This will have a significant impact on the prospect risk assessment of petroleum plays.

Continental exhumation triggered by partial melting at ultrahigh pressure

L. Labrousse et al., CNRS, UMR 7193, F-75005 Paris, France;

This paper by L. Labrousse of CNRS, Paris, and colleagues deals with the relationships between the occurrences of Ultra-High Pressure (UHP) metamorphic rocks (i.e. rocks buried in the Earth to more than 100 km deep and subsequently brought back to the surface) and migmatites (i.e. partially molten rocks). The comparison of the molten parts of migmatites associated with UHP rocks in the eroded mountain belt of the Norwegian Caledonides with glasses obtained by melting experiments on the same types of rocks show that partial melting may have begun when the rocks were at their maximal burial. Since it is known that partially molten rocks are weak material compared to solid rocks, it is therefore possible that the partial melting weakened the continental crust and helped it flowing upward by buoyancy.

Topset-dominated deltas: A new model for river delta stratigraphy

Douglas A. Edmonds et al., Dept. of Earth and Environmental Sciences, Boston College, 140 Commonwealth Avenue, Devlin Hall 213, Chestnut Hill, Massachusetts 02467, USA;

The most basic model for river delta stratigraphy came from the work of the great American geologist, G.K. Gilbert in the 19th century. Later work revealed that the "Gilbert-type" model, long thought to be universal, did not apply to all deltas. Since then, a second model has not been formally described. Here we propose a second basic model for river delta stratigraphy. Douglas A. Edmonds of Boston College and colleagues develop a geometric model that predicts the occurrence of "Gilbert-type deltas" and a new model, under different fluvial and basinal conditions. Their model predicts that, in contrast with Gilbert-type deltas, most deltas have channels that commonly incise degrees and the river feeding the delta has a discharge greater than 300 cubic meters per second. Field data from Gulf of Mexico deltas confirm the prediction that delta channels often incise into pre-delta sediment. Finally, Edmonds and colleagues propose a new stratigraphic model for deltas and suggest that future stratigraphic interpretation must distinguish between delta types to distinguish between environmental signals, like sea-level fall, and normal background incision.

Geological Society of America

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