Provenance of Des Moines lobe till records ice‐stream catchment evolution during Laurentide deglaciation     

BARBARA A. LUSARDI  CARRIE E. JENNINGS  KENNETH L. HARRIS 《Boreas: An International Journal of Quaternary Research》2011,40(4):585-597

Lusardi, B. A., Jennings, C. E. & Harris, K. L. 2011: Provenance of Des Moines lobe till records ice‐stream catchment evolution during Laurentide deglaciation. Boreas, 10.1111/j.1502‐3885.2011.00208.x. ISSN 0300‐9483. Mapping and analysis of deposits of the Des Moines lobe of the Laurentide Ice Sheet, active after the Last Glacial Maximum (LGM), reveal several texturally and lithologically distinct tills within what had been considered to be a homogeneous deposit. Although the differences between tills are subtle, minor distinctions are predictable and mappable, and till sheets within the area covered by the lobe can be correlated for hundreds of kilometres parallel to ice flow. Lateral till‐sheet contacts are abrupt or overlap in a narrow zone, coincident with a geomorphic discontinuity interpreted to be a shear margin. Till sheets 10 to 20 m thick show mixing in their lower 2 to 3 m. We suggest that: (i) lithologically distinct till sheets correspond to unique ice‐stream source areas; (ii) the sequence of tills deposited by the Des Moines lobe was the result of the evolution and varying dominance of nearby and competing ice streams and their tributaries; and (iii) in at least one instance, more than one ice stream simultaneously contributed to the lobe. Therefore the complex sequence of tills of subtly different provenances, and the unconformities between them record the evolution of an ice‐catchment area during Laurentide Ice Sheet drawdown. Till provenance data suggest that, after till is created in the ice‐stream source area, the subglacial conditions required for transporting till decline and incorporation of new material is limited.  相似文献   

Development of spatially diverse and complex dune-field patterns: Gran Desierto Dune Field, Sonora, Mexico   总被引：5，自引：0，他引：5  

CARRIE BEVERIDGE  GARY KOCUREK  RYAN C. EWING  NICHOLAS LANCASTER†  P. MORTHEKAI‡  ASHOK K. SINGHVI‡  SHANNON A. MAHAN§ 《Sedimentology》2006,53(6):1391-1409

The pattern of dunes within the Gran Desierto of Sonora, Mexico, is both spatially diverse and complex. Identification of the pattern components from remote‐sensing images, combined with statistical analysis of their measured parameters demonstrate that the composite pattern consists of separate populations of simple dune patterns. Age‐bracketing by optically stimulated luminescence (OSL) indicates that the simple patterns represent relatively short‐lived aeolian constructional events since ～25 ka. The simple dune patterns consist of: (i) late Pleistocene relict linear dunes; (ii) degraded crescentic dunes formed at ～12 ka; (iii) early Holocene western crescentic dunes; (iv) eastern crescentic dunes emplaced at ～7 ka; and (v) star dunes formed during the last 3 ka. Recognition of the simple patterns and their ages allows for the geomorphic backstripping of the composite pattern. Palaeowind reconstructions, based upon the rule of gross bedform‐normal transport, are largely in agreement with regional proxy data. The sediment state over time for the Gran Desierto is one in which the sediment supply for aeolian constructional events is derived from previously stored sediment (Ancestral Colorado River sediment), and contemporaneous influx from the lower Colorado River valley and coastal influx from the Bahia del Adair inlet. Aeolian constructional events are triggered by climatic shifts to greater aridity, changes in the wind regime, and the development of a sediment supply. The rate of geomorphic change within the Gran Desierto is significantly greater than the rate of subsidence and burial of the accumulation surface upon which it rests.  相似文献   

CHANGES IN ICE-MARGIN PROCESSES AND SEDIMENT ROUTING DURING ICE-SHEET ADVANCE ACROSS A MARGINAL MORAINE     

PETER G. KNIGHT  CARRIE E. JENNINGS  RICHARD I. WALLER  ZOE P. ROBINSON 《Geografiska Annaler: Series A, Physical Geography》2007,89(3):203-215

Advance of part of the margin of the Greenland ice sheet across a proglacial moraine ridge between 1968 and 2002 caused progressive changes in moraine morphology, basal ice formation, debris release, ice‐marginal sediment storage, and sediment transfer to the distal proglacial zone. When the ice margin is behind the moraine, most of the sediment released from the glacier is stored close to the ice margin. As the margin advances across the moraine the potential for ice‐proximal sediment storage decreases and distal sediment flux is augmented by reactivation of moraine sediment. For six stages of advance associated with distinctive glacial and sedimentary processes we describe the ice margin, the debris‐rich basal ice, debris release from the glacier, sediment routing into the proglacial zone, and geomorphic processes on the moraine. The overtopping of a moraine ridge is a significant glaciological, geomorphological and sedimentological threshold in glacier advance, likely to cause a distinctive pulse in distal sediment accumulation rates that should be taken into account when glacial sediments are interpreted to reconstruct glacier fluctuations.  相似文献