Thomas C. Gustavson

A depositional model for outwash, sediment sources, and hydrologic characteristics, Malaspina Glacier, Alaska: A modern analog of the southeastern margin of the Laurentide Ice Sheet

The Malaspina Glacier on the southern coast of Alaska is a partial analog of the late Wisconsinan Laurentide Ice Sheet that occupied New England and adjacent areas. Ice lobes of the Malaspina are similar in size to end moraine lobes in southern New England and Long Island. Estimated ablation rates, surface slopes, and meltwater discharge per unit of surface area for the Laurentide Ice Sheet are comparable to measured ablation rates, surface slopes, and meltwater discharge rates for the Malaspina Glacier. Meltwater moves from the surface of the Malaspina down-glacier and toward the bed of the glacier along intercrystalline pathways and through a series of tunnels. Regolith beneath the glacier, which is eroded and transported to the margin of the glacier by subglacial and englacial streams, is the source of essentially all fluvial and lacustrine deposits on the Malaspina Foreland. By analogy, a similar hydrologic system existed at the southeastern margin of the Laurentide Ice Sheet. Subglacial regolith, which was eroded from beneath the ice sheet by meltwater, was the source of most stratified sediment deposited in New England and adjacent areas during the late Wisconsinan. Similarly, Wisconsinan ice-contact landforms in New England were built by the same processes that are constructing landforms composed of stratified sediments in contact with the Malaspina Glacier. For the Malaspina Glacier and the Laurentide Ice Sheet, therefore, we reject the concept of the “dirt machine” by which debris near the base of the glacier is carried to the surface of the glacier along shear planes and then washed off the surface to form ice-contact stratified deposits.

Depositional facies of the Miocene-Pliocene Ogallala Formation, northwestern Texas and eastern New Mexico

Deposition of the basal fluvial sediments of the Miocene-Pliocene Ogallala Formation in western Texas and eastern New Mexico was controlled by topography on the underlying erosional surface. Paleovalley-fill facies consist of gravelly and sandy braided-stream deposits interbedded with and overlain by eolian sediments deposited as sand sheets and loess. Uplands on the pre-Ogallala erosional surface are overlain primarily by similar eolian sediments. Calcic paleosols, consisting mostly of glaebules and rhizoconcretions of CaCO/sub 3/, occur throughout the eolian facies. Massive to laminated and locally pisolitic, brecciated, and recemented pedogenic calcretes occur primarily near or at the top of the Ogallala Formation. Eolian facies preserve numerous superposed calcretes and calcic paleosols, reflecting slow episodic aggradation on a savannah or grassland under arid to subhumid climatic conditions. The change from fluvial to mostly eolian sedimentation probably resulted from diversion of streams that deposited fluvial sediments of the Ogallala Formation to form the Pecos and Canadian rivers. Source areas for eolian sediments may initially have been flood plains of Ogallala braided streams and later the flood plains of the newly formed Pecos and Canadian rivers. 38 references.

Geomorphic development of the Canadian River Valley, Texas Panhandle: An example of regional salt dissolution and subsidence

Development of the Canadian River Valley in the Texas Panhandle resulted mostly from regional subsidence following dissolution of Permian bedded salts. Salts of the Clear Fork, Glorieta, San Andres, and Seven Rivers Formations have undergone dissolution along the margins of the Palo Duro, Dalhart, and Anadarko Basins. The Canadian River Valley follows a zone of subsidence for

Sedimentation on gravel outwash fans, Malaspina Glacier Foreland, Alaska

ABSTRACT Three depositional facies occur on the outwash fans of Fountain Stream and Alder Stream, which presently drain portions of the Malaspina Glacier along the northeastern Gulf of Alaska. Deposition of longitudinal bars occurs during high-flow stage and consists of plane-bedded, imbricate, very poorly sorted pebble and cobble gravels. The sediment of the bars decreases in mean size downstream. Some bars terminate in avalanche slip faces of poorly sorted, silty, medium to coarse sand. Paleocurrent directions suggest that stream flow diverges across bar surfaces. Deposition in channels occurs in two morphologically distinct areas, riffles and pools. Sediment deposited in pools consists primarily of ripple cross-laminated, poorly sorted, silty, medium to coarse sand, commonly capped by draped lamination. Ripple crests vary from straight to cuspate. Sediment in riffles consists of gravel laid down under upper flow regime conditions as transverse ribs and stone cells. Both apparently are relict antidune bedforms. During late-stage flow, thin patches of horizontally bedded sand are deposited between transverse ribs and adjacent stone-cell borders. The longitudinal bar facies and channel pool facies are common in cutbank sections. The channel riffle facies was not recognized in cutbank sections. Scour pits, commonly observed on some bar surfaces, are produced by currents scouring around grounded ice blocks. Ice-block trails are produced by ice blocks dragged through soft sediment. Neither of these sedimentary structures have been reported in the literature. If found in ancient sediments, both structures would indicate a nearby source of ice.

Lithostratigraphy and geochronology of fills in small playa basins on the Southern High Plains, United States

Playa basins are small depressions (typically ≤1.5 km 2 ) on the Southern High Plains of northwestern Texas and eastern New Mexico. There are about 25 000 playas in the region; they lie on the Blackwater Draw Formation (Pleistocene), a widespread eolian deposit, and locally on the Ogallala Formation (Miocene‐ Pliocene). Understanding the lithostratigraphy and chronostratigraphy of the fill in the basins is important because it should (1) provide clues to the origin and evolution of playas, which have been long debated; (2) yield a paleoenvironmental record for the region; and (3) aid in understanding the history and future of the regional aquifer because playas are the principal source of recharge. Data from 19 playa basins, combined with published data from 4 other basins, show that the basin fill is composed of six distinctive facies: (1) lacustrine mud; (2) lacustrine carbonate; (3) lacustrine delta deposits; (4) eolian sand and silt; (5) eolian loam; and (6) accretionary eolian deposits (Blackwater Draw Formation). Mud deposited under ponded conditions is the most common facies and is the surficial deposit on the floors of most playas, often producing Vertisols. The carbonate was precipitated under lacustrine conditions and is another common facies and surface deposit. Delta deposits are common near the basin margins. Well-sorted layers of eolian sand and silt and poorly sorted eolian loam occur locally above, within, or below the lacustrine deposits. The modern basins in all study areas are locally or completely inset into the Blackwater Draw Formation, supporting the interpretation that the basins are at least in part erosional features. In larger basins with thicker fills, generally coincident with thicker Blackwater Draw Formation, the formation interfingers with the lacustrine fill. Dating is based on radiocarbon ages from the fill in 12 basins and from lunettes adjacent to 5 basins. All dated basins were present at the end of the Pleistocene and some were present in some form throughout the Pleistocene. Lacustrine mud and other clastic deposits accumulated in the late Quaternary and locally much earlier, showing that at least some basins contained water throughout the time of human occupation of the region. Dating of eolian sediments supports other data indicative of aridity and wind deflation in the early and late Holocene. The lacustrine carbonate is late Pleistocene or older and its paleoenvironmental significance is unknown. These lithostratigraphic and chronostratigraphic relationships show that some basins have a prolonged history as depressions, persisting in more or less the same location as the High Plains surface aggraded by eolian addition (Blackwater Draw Formation) throughout the Pleistocene. Sizes of the basins varied through time as they were encroached upon by the Blackwater Draw Formation, enlarged by fluvial, lake margin, and eolian erosion, were filled and reexposed, or were buried. Some basins are newly formed on the High Plains surface and have no apparent predecessors. The only evidence for subsidence beneath the basins is gently warped fill in the basins on the northern margin of the region, known to be affected by salt dissolution in Paleozoic bedrock. Pedogenic carbonates typically are absent from or beneath the basin fill, due to focused recharge through the basins. Playa basins probably have been a ubiquitous component of the High Plains landscape through much of the Quaternary.

Origin of Satin Spar Veins in Evaporite Basins

ABSTRACT Satin spar (fibrous gypsum) veins occur in rocks overlying evaporites in the Amadeus Basin, Australia; Appalachian Basin, USA; Cheshire Basin, England; Elk Point Basin, Canada; Palo Duro Basin, USA; Paradox Basin, USA, and Zechstein Basin, England. These antitaxial veins, which are characterized by central partings and near-vertical fibers, fill horizontal and inclined fractures. Most satin spar veins occur in highly fractured or brecciated clastic strata that overlie or are associated with bedded halite, anhydrite, and gypsum. Saline springs are commonly present, indicating that halite dissolution is active. The similarities in morphology and occurrences of these veins suggest a common genesis. Satin spar veins result from several simultaneously active processes. Recharge of low-salinity surface water results in dissolution of shallow ( 200 m to 750 m deep) halite, development of cavernous porosity, and formation of extensional fractures in the rock column overlying salt-dissolution zones. Greater solubility of anhydrite than gypsum at low temperatures and at salinities below halite saturation causes hydration of anhydrite to gypsum, which takes place without significant volume change. Once groundwater is saturated with respect to gypsum, any further anhydrite hydration and solution must result in supersaturation, and the excess CaSO4 carried by ground water flowin from dissolution zones precipitates as gypsum in open, high-permeability fractures.

Buried Vertisols in lacustrine facies of the Pliocene Fort Hancock Formation, Hueco Bolson, West Texas and Chihuahua, Mexico

Buried Vertisols, which are commonly preserved in smectite-rich clay and sandy clay facies of the upper Tertiary Fort Hancock Formation, are products of the late Tertiary depositional and paleoclimatic environment in the Hueco Bolson, West Texas, United States, and Chihuahua, Mexico. Fort Hancock clayey facies are in part laminated and contain abundant desiccation cracks and locally gypsum beds, suggesting deposition in an ephemeral or playa lake. Buried Vertisols apparently resulted from repeated shrink/swell cycles due to drying and wetting of expansive clays in the ephemeral lake basin. Lacustrine facies are interbedded with gravel, sand, and sandy mud facies that were likely deposited as alluvial fans and fan deltas at the lake margin. Calcic soil horizons, which form mostly in arid to subhumid modern climates and consist of CaCO 3 nodules and filaments, are present in most buried Vertisols of the Fort Hancock Formation. Collectively, buried Vertisols with calcic soil horizons and ephemeral-lake and alluvial-fan deposits suggest that the Fort Hancock Formation accumulated in an arid to semiarid climate.

Bathymetry and Sediment Distribution in Proglacial Malaspina Lake, Alaska

ABSTRACT Glaciolacustrine rhythmites are being deposited in proglacial Malaspina Lake, Alaska. Fathometer profiles of the lake bottom depict irregular subglacial topography little modified by lacustrine sedimentation and areas of flat fleatureless topography that are underlain by sequences of graded lacustrine sediments. Turbidity currents were recorded along the ice-contact margin of the lake and adjacent to inflowing streams. Cores taken from the lake bottom contain rhythmites whose sandy and silty portions consist of normal and reverse graded laminae. The distribution of sand and silt on the lake bottom indicates that sand and silt content of the rhythmites decreases away from known sediment sources--the streams and the areas where turbidity currents and interflows were observed near the margin of the Malaspina Glacier. The lake bottom topography, the presence of turbidity currents, rhythmites with current-bedded sands and silts, and the sediment distribution on the lake bottom all suggest that the coarse fraction of rhythmites is deposited from turbidity currents. The clay fraction on the other hand is deposited from suspension when the turbidity currents cease. Examination of Fathometer profiles indicates that mass movement of lake bottom sediments is common resulting from the collapse of lake sediments over melting ice. Ice taken from the lake bottom in two cores also suggests that parts of Malaspina Lake are still underlain by glacial ice.

Structural influences on geomorphic processes and physiographic features, Texas Panhandle: Technical issues in siting a nuclear-waste repository

Deaf Smith County in the Texas Panhandle is being considered by the U.S. Department of Energy as a possible high-level nuclear-waste disposal site. Among the geologic issues being considered are the timing and processes of salt dissolution and related subsidence. Movement along northeast-trending Paleozoic faults in the Texas Panhandle appears to have influenced deposition during the late Paleozoic. Salt of the Upper Permian Seven Rivers Formation appears to have been dissolved preferentially along the same northeast trend. Structural features of post-Seven Rivers strata as well as segments of surface stream valleys overlie and are parallel to the zone of preferential salt dissolution. The association of structure, depositional features, dissolution, and physiographic features, ranging in age from Paleozoic to Neogene, suggests a persistent structural influence.