SPACE‐TIME MODELING OF GRIZZLY BEARS*

ABSTRACT. An appropriate conceptual leap can be made from human geography to physical geography by applying space‐time continuum modeling to zoogeomorphology—the alteration of the landscape by animals. Drawing on the work of Torsten Hägerstrand, we propose a space‐time continuum model of grizzly bears, a geomorphic agent. Through both data‐driven and conceptual applications, the model traces where and when grizzly bears are likely to wield a geomorphic impact. The model further demonstrates that geographers have much to gain by merging concepts from human and physical geography.     

Flash floods and panic in the fossil record: a tale of 25 titanotheres

The Eocene rocks of southern Wyoming have provided a spectacular discovery of fossil bones and teeth. The remains of 25 titanotheres were found all together in one place less than 100 m square. Some of these horse-like mammals were young, some very old; and they were part of a herd. The geological and social evidence indicates that they perished together in a flash flood.     

Variable Variscan thermal rejuvenation in the St Malo region, Cadomian Orogen, France: evidence from 40Ar/39Ar mineral ages

Abstract The St Malo region in north-west France contains migmatites and anatectic granites derived by partial melting of metasedimentary protoliths during Cadomian orogenesis at c. 540 Ma. Previously reported Rb–Sr model ages for muscovite and biotite range from c. 550 to c. 300 Ma, and suggest variable resetting of mineral isotopic systems. These rocks display microscopic evidence for variably intense Cadomian intracrystalline plastic strain but record no obvious evidence of penetrative Palaeozoic regional deformation. ⁴⁰Ar/³⁹Ar mineral ages have been determined to evaluate better the extent, timing and significance of Palaeozoic overprinting. Eleven muscovite concentrates and one whole-rock phyllite have been prepared from various units exposed in the St Malo and adjacent Mancellian regions. In the Mancellian region, muscovite from two facies of the Bonnemain Granite Complex record ⁴⁰Ar/³⁹Ar plateau ages of c. 527 and 521 Ma. An internally discordant ⁴⁰Ar/³⁹Ar release spectrum characterizes muscovite from protomylonitic granite within the Cadomian Alexain-Deux Evailles-Izé Granite Complex, and probably records the effects of Variscan displacement along the North Armorican Shear Zone. Muscovite concentrates from anatectic granite and from Cadomian mylonites along ductile shear zones within the north-western sector of the St Malo region exhibit internally discordant ⁴⁰Ar/³⁹Ar release spectra which suggest variable and partial late Palaeozoic rejuvenation. By contrast, muscovite concentrates from samples of variably mylonitic Brioverian metasedimentary rocks exposed within the south-eastern sector of the St Malo region display internally concordant apparent age spectra which define plateaux of 326–320 Ma. A whole-rock phyllite sample from Brioverian metasedimentary rocks exposed along the eastern boundary of the St Malo region displays an internally discordant argon release pattern which is interpreted to reflect the effects of a partial late Palaeozoic thermal overprint. Muscovite from the Plélan granite, part of the Variscan Plélan-Bobital Granite Complex, yields a ⁴⁰Ar/³⁹Ar plateau age of c. 307 Ma. The ⁴⁰Ar/³⁹Ar results indicate that Cadomian rocks of the St Malo region have undergone a widespread and variable Palaeozoic (Carboniferous) rejuvenation of intracrystalline argon systems which apparently did not affect the Mancellian region. This rejuvenation was not accompanied by penetrative regional deformation, and was probably of a static thermal–hydrothermal origin. The heat source for rejuvenation was probably either the result of heating during Variscan extension or advection from Variscan granites which are argued to underlie the St Malo region.     

Depositional record of an avulsive fluvial system controlled by peat compaction (Neogene, Most Basin, Czech Republic)

This paper illustrates the response of a fluvial depositional system to the interplay between peat compaction and clastic sediment supply, at a range of spatial and temporal scales, as documented by extensive exposures in an open-cast mine in the Most Basin, part of the Oligo-Miocene Ohře Rift (Eger Graben) basin system in the Czech Republic. The Most Basin is characterized by the occurrence of a number of phenomena resulting from syn- and post-depositional interactions between clastic sedimentary systems and the underlying accumulation of organic material that was the precursor of the main lignite seam of up to 45 m thickness. The studied strata are interpreted as deposits of an avulsive, mixed-load fluvial system. The large-scale depositional architecture documents an existence of at least five stratal packages up to 1500 m wide and up to several tens of metres thick, representing a record of long-term evolution of a clastic floodplain bordered by accumulating peat. Within each of the packages, several small-scale channel-belts were documented. Individual packages are separated by carbonaceous mudstones indicating a period of reduced clastic input and interpreted as due to avulsion of the fluvial channels out of the floodplain limit. Two main, mutually linked, processes controlled the evolution of the studied fluvial system: (i) syndepositional compaction of the underlying peat and (ii) avulsions of the channels away from the original floodplain, resulting in formation of a new floodplain. The processes which caused the channels of the Hrabák fluvial system to reach the avulsion threshold were: (i) decrease of rate of creation of accommodation leading to increased sinuosity and thus to a decreased channel slope, and (ii) cross-floodplain tilting of the channel belt caused by differential compaction of underlying organic-rich substratum.     

Sea-level and tectonic control of middle to late Pleistocene turbidite systems in Santa Monica Basin, offshore California

Small turbidite systems offshore from southern California provide an opportunity to track sediment from river source through the turbidity‐current initiation process to ultimate deposition, and to evaluate the impact of changing sea level and tectonics. The Santa Monica Basin is almost a closed system for terrigenous sediment input, and is supplied principally from the Santa Clara River. The Hueneme fan is supplied directly by the river, whereas the smaller Mugu and Dume fans are nourished by southward longshore drift. This study of the Late Quaternary turbidite fill of the Santa Monica Basin uses a dense grid of high‐resolution seismic‐reflection profiles tied to new radiocarbon ages for Ocean Drilling Program (ODP) Site 1015 back to 32 ka. Over the last glacial cycle, sedimentation rates in the distal part of Santa Monica Basin averaged 2–3 mm yr^?1, with increases at times of extreme relative sea‐level lowstand. Coarser‐grained mid‐fan lobes prograded into the basin from the Hueneme, Mugu and Dume fans at times of rapid sea‐level fall. These pulses of coarse‐grained sediment resulted from river channel incision and delta cannibalization. During the extreme lowstand of the last glacial maximum, sediment delivery was concentrated on the Hueneme Fan, with mean depositional rates of up to 13 mm yr^?1 on the mid‐ and upper fan. During the marine isotope stage (MIS) 2 transgression, enhanced rates of sedimentation of > 4 mm yr^?1 occurred on the Mugu and Dume fans, as a result of distributary switching and southward littoral drift providing nourishment to these fan systems. Longer‐term sediment delivery to Santa Monica Basin was controlled by tectonics. Prior to MIS 10, the Anacapa ridge blocked the southward discharge of the Santa Clara River into the Santa Monica Basin. The pattern and distribution of turbidite sedimentation was strongly controlled by sea level through the rate of supply of coarse sediment and the style of initiation of turbidity currents. These two factors appear to have been more important than the absolute position of sea level.     

Modelling of turbidity currents on Navy Submarine Fan, California Continental Borderland

Several Holocene turbidites can be correlated across much of Navy Fan through more than 100 sediment core localities. The uppermost muddy turbidite unit is mapped throughout the northern half of the fan; its volume, grain-size distribution and the maximum height of deposition on the basin slopes are known. These parameters can be related to the precise channel morphology and mesotopography revealed by deep-tow surveys. Thus there is sufficient information to estimate detailed flow characteristics for this turbidity current as it moved from fan valley to distal basin plain. On the upper fan, the gradient and the increasing downstream width of the channel and only limited flow overspill suggest that the flow had a Froude number close to 1.0. The sediment associated with the channel indicates friction velocities of about 0.06 m s^?1 and flow velocities of about 0.75 m s^?1. Using this flow velocity and channel dimensions, sediment concentration (～2×10^?3) and discharge are estimated, and from a knowledge of the total volume of sediment deposited, the flow duration is estimated to be from 2 to 9 days. It is shown that the estimates of Froude number, drag coefficient, and sediment concentration are not likely to vary by more than a factor of 2. On the mid-fan, the flow was much thicker than the height of the surface relief of the fan and it spread rapidly. The cross-flow slope, determined from the horizontal extent of turbidite sediment, is used to estimate flow velocity, which is confirmed by consideration of both sediment grain size and rate of deposition. This again allows sediment concentration and discharge to be estimated. The requirements of flow continuity, entrainment of water during flow expansion, and observed sediment deposition provide checks on all these estimates, and provide an integrated picture of the evolution of the flow. The flow characteristics of this muddy turbidity current are well constrained compared to those for more sand-rich late Pleistocene and early Holocene turbidity currents on the fan.