Bedrock Depressions Research Articles

Relative instability of colluvium‐filled bedrock depressions

AbstractRegolith landslides commonly occur in colluvium‐filled bedrock depressions rather than on ridges, spurs, or sites with planar bedrock surfaces. Drained direct shear tests of colluvium from bedrock depressions in Wellington, New Zealand, indicate the presence of cohesion in all samples, suggesting that thickness of accumulated deposits may become a limiting factor for stability. Critical depths for a range of slope angles were calculated (assuming conditions of infinite limiting equilibrium) and compared to the depth/angle relationships for depression and interdepression regolith occurring in the field. The results indicate that, under saturated conditions, interdepression regolith is not thick enough to permit the development of landslides. Colluvium‐filled bedrock depressions, on the other hand, generally exceed critical depth and must be considered susceptible to landsliding in the absence of any significant root cohesion.The age of deposits and the abundance of unfailed colluvium‐filled bedrock depressions suggest that they have been inactive for most of post glacial time. Stability analysis, verified by observing the occurrence of landslides, indicates that recent instability has been brought about by deforestation.

Recent Geological History and Modern Sedimentary Processes Along an Incipient, Low-Energy, Epicontinental-Sea Coastline: Northwest Florida

ABSTRACT A major portion of Florida's Gulf of Mexico coastline is a siliciclastic, sand-starved, low-wave-energy system dominated by marshes that face the open sea. Because of its location at the center of an enormous (350,000 km2), flooded, broad, flat, ancient carbonate platform, this coastline can be viewed as the margin of an incipient epicontinental sea. Within a 65-km sector along this 300-km-long, open-marine marsh coast, four distinctly different morphological sectors have been identified: 1) berm-ridge marsh shoreline, 2) marsh peninsula shoreline, 3) marsh archipelago shoreline, and 4) shelf embayment shoreline. The underlying Paleogene limestone bedrock topography results from karstification and dissolution processes. This antecedent topography and the distribution of actively discharging freshwater springs affect sedimentary processes, facies, and stratigraphic units. The morphology and processes within the four coastal sectors vary as a function of a northerly decrease in the thickness of relict Pleistocene quartz-sand cover and an increase in the complexity and relief of the underlying bedrock topography. Each of the four coastal sectors has a unique distribution of sedimentary products. In addition, each has responded differently to rising sea level--a process that continues to control the spatial distribution of organisms and sediments. However, the sedimentary lithosomes in all four coastal sectors share a similar fate during landward shoreline translation. Within the inner continental shelf, very little of the marsh stratigraphy or large oyster bioherms is preserved. Only those sediments that have accumulated within bedrock depressions or sinkholes have the best chance for long-term retention. Thus, a condensed stratigraphic section is produced. Since only thin and discontinuous sediment accumulations are preserved, the relief and texture of the bedrock surface due to chemical etching and bioerosion must also be used as clues to the recognition of similar coastal systems in ancient settings.

Till sampling in the Casa-Berardi gold area, Quebec: A case history in orientation and discovery

Sauerbrei, J.A., Pattison, E.F. and Averill, S.A.,, 1987. Till sampling in the Casa-Berardi gold area, Quebec: A case history in orientation and discovery. In: R.G. Garett (Editor), Geochemical Exploration 1985. J. Geochem. Explor., 28: 297-314. Inco Limited discovered gold in northwestern Casa-Berardi township, Quebec, in April 1981. The discovery, named the Golden Pond Zone, is located under approximately 45 m of overburden.Diamond drilling at Golden Pond up to August 1983 indicated two main zones of Au mineralization on either side of a major east-west-trending graphitic fault zone. The mineralization consists of pyrite-arsenopyrite-native Au-bearing quartz-carbonate veins in carbonatized Archaen sediments and volcanics. A reverse-circulation overburden drilling program was undertaken in the vicinity of the Golden Pond zone in December 1983. The program, under the management of Overburden Drilling Management of Ottawa, was designed to test for glacial dispersal of the known mineralization and thereby to evaluate the effectiveness of till sampling as an Au exploration method in the Casa-Berardi area. Gold and As anomalies related to the Golden Pond zone are found mainly in heavy-mineral concentrates of the lower till that lines the bedrock depression which is spatially associated with bedrock mineralization. These anomalies occur south of the known mineralization and constitute an example of a glacial dispersal train. Most of the till gold grains are very fine (50-150 microns). The gold grains in the till 100 m south of the Golden Pond zone are delicate while the majority of those encountered 400 m to the south are either irregular or abraded, indicating transport over greater distances. Following the success of orientation survey, till sampling was used as a screening tool to evaluate geophysical and/or stratigraphic targets east and west of the Golden Pond zone prior to diamond drilling. During the initial sampling program in early 1984, targets were commonly tested at intervals of either 300 m or 400 m along strike and 25-100 m down ice. Closer-spaced sampling was undertaken to further define anomalous dispersal trains prior to diamond drilling. Two reversecirculation holes drilled 300 m apart in April 1984 to test the continuation of the Golden Pond geophysics encountered an anomalous gold-arsenopyrite dispersal train 2.5 km to the east. Followup till sampling in June 1984 further defined the anomalous train. The heavy-mineral concentrate from the bottom till sample of a hole 100 m east of the earlier anomalous hole contained approximately 700 very delicate gold grains, mostly in the 50-100 micron size range, and was concluded to be at source. Diamond drilling in July 1984 confirmed the existence of the Golden Pond East zone. 0375-6742/87/$03.50 © 1987 Elsevier Science Publishers B.V.

The Tvären Bay structure, an astrobleme in southeastern Sweden

Abstract Continuous seismic reflection profiling has revealed a 2000 m wide, almost circular depression in the crystalline bedrock of Tvaren Bay. The depression extends more than 200 m below the present sea level. Originally, it was probably 250–300 m deep. The physical parameters of the structure, which forms a foreign element in the tectonic regime of the Baltic coastal area, are consistent with those of known impact structures. Inclined sedimentary rocks of an assumed Ordovician age are unevenly distributed within the depression, partially smoothening the topographic relief of the structure. A late Lower or early Middle Ordovician time for the meteorite impact is suggested. Drillings into the ‘wall’ surrounding the impact structure and into the sedimentary bedrock inside the depression are proposed as a further step in this investigation.

Foundation problems on limestone: A case history from the Carboniferous Limestone at Chepstow, Gwent

Summary Sediment filled bedrock depressions in the Carboniferous Limestone were revealed during construction of a supermarket in Chepstow. They were interpreted as swallets. An initial site investigation did not reveal the features, which were less than 10 m across and about 8 m deep. They were not suspected at the desk study stage since the site was on a valleyside location and did not lie on an obvious abandoned drainage route. The features were investigated in less than one week and the investigation and redesign of foundations did not incur any delay to the contract.

Buried ancestral drainage between Lakes Erie and Ontario

The bedrock topography for the eastern Niagara Peninsula of Ontario and for adjacent areas of New York was reconstructed using more than 7,600 well records and geophysical data. This topography reveals an elaborate system of buried channels dominated by three main passageways connecting Lake Erie to Lake Ontario. These channels are the Erigan, Crystal Beach, and St. Davids. The Erigan, with a depth near 150 ft below Lake Erie water level, precludes the existence of a lake in the Erie basin. North of the Niagara Escarpment, the Erigan channel bifurcates into three outlets that extend into Lake Ontario. The extensive widths and differences in elevation of the outlet valleys suggest that the Erigan channel was active at least three times. Elevations in two of the outlets indicate that water levels in the ancestral Lake Ontario were similar to those of the present-day lake. The third outlet requires water levels at least 30 ft lower than present conditions. The Crystal Beach channel originates west of Buffalo and trends north along part of the present-day course of the Niagara River. At Niagara Falls, it changes to a southwesterly direction to join the Erigan channel near its southern end. The Crystal Beach system received inflow from Lake Erie through at least 4 inlets with bed elevations only 10–30 ft below the present Lake Erie water level. This suggests the existence of an ancestral Lake Erie similar to present-day conditions. Limited retreat in the waterfall of the Erigan channel indicates that during the active period of the Crystal Beach and Erigan drainage systems, only the Lake Erie watershed emptied through the Niagara Peninsula. The Upper Great Lakes may have drained through the buried Laurentian valley between Georgian Bay and Lake Ontario. Drainage of the Upper Great Lakes through the Niagara Peninsula may have first occurred through the St. Davids buried channel. The limited retreat of the St. Davids waterfall suggests a history of only a few thousand years. Its channel above the Niagara Escarpment probably occupied part of the bedrock depression formed by the Crystal Beach drainage system, but at a higher elevation.

Sedimentary framework of Penobscot Bay, Maine

Analyses of seismic-reflection profiles, along with previously collected sediment samples and geologic information from surrounding coastal areas, outline the characteristics, distribution, and history of the strata that accumulated within Penobscot Bay, Maine, during the complex period of glaciation, crustal movement, and sea-level change since late Wisconsinan time. Sediments that overlie the rugged, glacially eroded surface of Paleozoic bedrock range in thickness from near zero to more than 50 m and consist of four distinct units. 1. (1) Massive to partly stratified, coarse-grained drift forms thin (< 15 m) isolated patches along the walls and floors of bedrock troughs and constitutes a thick (up to 30 m), hummocky end moraine in the central part of the bay. The drift was deposited by the last ice sheet between 12,700 and 13,500 years ago during deglaciation and coastal submergence (due to crustal depression). 2. (2) Well-stratified, fine-grained glaciomarine deposits are concentrated in bedrock depressions beneath the main passages of the bay. During the period of ice retreat and marine submergence, these sediments settled to the sea floor, draped the irregular underlying surface of bedrock or drift, and accumulated without disturbance by physical or biologic processes. 3. (3) Heterogeneous fluvial deposits fill ancestral channels of the Penobscot River beneath the head of the bay. The channels were incised during a −40 m postglacial low stand of sea level (due to crustal rebound) and later were filled as base level was increased during Holocene time. 4. (4) Muddy marine sediments, which are homogeneous to weakly stratified and rich in organic matter, blanket older deposits within bathymetric depressions in the middle and lower reaches of the bay and cover a pronounced, gently dipping, erosional unconformity in the upper reach. These sediments were deposited during the Holocene transgression as sea level approached its present position and the bay became deeper. Late Wisconsinan and Holocene sedimentation in Penobscot Bay has smoothed the sea floor, but it has not completely obscured the ice-sculptured bedrock topography.

The Goose Arm karst, Newfoundland, Canada

Goose Arm karst comprises 200 km 2 of deformed carbonates with relief of 60–350 m, blanketed with carbonate-rich till of the Last Glaciation. Principal karst landforms are 50 large bedrock depressions, implying mature well-integrated karst development. Modern groundwater systems are short, young, non-integrated. The anomaly is explained by injection of glacial till into a paleo-aquifer, clogging it. Buffering by till also yields a remarkably uniform bicarbonate water chemistry, but groundwater temperatures are anomalous.

Large-Scale Numerical Modelling of the Antarctic Ice Sheet

A large-scale dynamic numerical model of the Antarctic ice sheet has been developed to study its present state of ice flow and mass balance as well as its response to long-term changes of climate or sea-level.The flow of ice over a two-dimensional grid is determined from the ice thickness, the basal shear stress, the bedrock depth, and ice flow parameters derived from velocities of existing ice sheets. The change in ice thickness with time is governed by the continuity equation involving the ice flux divergence and the ice accumulation or ablation. At the ice sheet seaward boundary, a floating criterion and floating ice thinning rate apply. Bedrock depression with a time-delayed response dependent on the history of the ice load is also included.A 61 × 61 point grid with 100 km spacing has been used to represent the ice-sheet surface, bedrock, and accumulation rate. The model has been used to simul a te the growth of the present ice sheet and i ts reaction to changes of sea-level, bedrock depression, accumulation rate, ice flow parameters, and the iceshelf thinning rate.Preliminary results suggest that the present ice sheet is not in equilibrium but rather is still adjusting to changes of these parameters.

Large-Scale Numerical Modelling of the Antarctic Ice Sheet

A large-scale dynamic numerical model of the Antarctic ice sheet has been developed to study its present state of ice flow and mass balance as well as its response to long-term changes of climate or sea-level.The flow of ice over a two-dimensional grid is determined from the ice thickness, the basal shear stress, the bedrock depth, and ice flow parameters derived from velocities of existing ice sheets. The change in ice thickness with time is governed by the continuity equation involving the ice flux divergence and the ice accumulation or ablation. At the ice sheet seaward boundary, a floating criterion and floating ice thinning rate apply. Bedrock depression with a time-delayed response dependent on the history of the ice load is also included.A 61 × 61 point grid with 100 km spacing has been used to represent the ice-sheet surface, bedrock, and accumulation rate. The model has been used to simul a te the growth of the present ice sheet and i ts reaction to changes of sea-level, bedrock depression, accumulation rate, ice flow parameters, and the iceshelf thinning rate.Preliminary results suggest that the present ice sheet is not in equilibrium but rather is still adjusting to changes of these parameters.

Environments and Dynamics of Clastic Sediment Dispersal Across Cambrian of Grand Canyon: ABSTRACT

End_Page 1004------------------------------The transgressive Grand Canyon Cambrian contains basal fluvial sands (Tapeats Sandstone) and overlying shallow marine sands and shales (Bright Angel Shale) in which textures and structures permit a detailed reconstruction of clastic sediment dispersal across a broad cratonic margin. Dominantly trough cross-bedded basal Tapeats Sandstone, containing buried regolith in bed-rock depressions, a low-variance, unimodal paleocurrent trend down the regional paleoslope, and well to moderately sorted medium to coarse-grained sands, records pre-vegetation bed-load fluvial sedimentation. Bed-load sands (> 200 mµ) mature from arkosic to orthoquartzitic within 5 to 10 m of the base. Finer, suspension-transported sands remain subarkosic through the entire 250 to 500 m of Cambrian section implying rapid dispersal. The Bright Angel Shale is composed of shallowing upward sedimentary cycles 1 to 6 m thick. Lower parts of the cycles are thinly interbedded fine sand and shale layers whose sedimentary structures and textures record level-bottom storm resuspension, dispersal, and deposition (sand layers) alternating with quiescent periods of finer silt and clay dispersal and accumulation. Bed-load transported sands are absent. Sand bed thickness up to a maximum (5 to 20 cm) represents variation in storm intensities with maximum thicknesses controlled by water depth, which limits volume in suspension. Upper parts of cycles are cross-laminated fine sands with glauconite and U-shaped or vertical burrows. These represent shoaling phases with increased bottom agitation, decreased sedimentation rate, and winnowing bypass of silts and clays. Organism structures, tadpole ripples, and rare mud cracks record shallowing. Cycle caps are 1 to 20-cm thick layers of medium to very coarse quartz sand, hematitic ooids, phosphatic, and dolomite cemented clasts that record phases of emergence and complete bypass of all suspension load. Recently recognized dunes 3 to 5 m in height provided bed load dispersal at the close of sedimentation cycles. Structures and textures suggest dunes are eolian. End_of_Article - Last_Page 1005------------

Stratigraphy and chronology of receding barrier‐beach deposits on the northern Illawarra coast of New South Wales

Abstract Four Quaternary units occupying a shallow bedrock depression have been delineated from outcrop exposures and auger drilling on Sandon Point Beach, North Bulli, 60 km south of Sydney, N.S.W. In ascending stratigraphic order these units are: a basal muddy sand (2.2+ m) of fluvial origin; a mottled mud (3.5 m) with common iron oxides, representing alluvial deposits with a superimposed soil profile developed during a period of lower sea level; a sandy mud (3.9 m) containing estuarine shell fossils, small tree stumps (including one mangrove) and abundant wood fragments; and, at the top, a pebble layer overlain by beach and dune sands, all partly indurated with iron oxides (2.3 m). 14C dating in situ tree stumps, shell material and wood fragments from the estuarine sandy mud unit between mean sea level and +1.45 m provided ages ranging from 7520 ±. 150 to 6350 ± 100 years B.P. The age of this unit is supported by a date on in situ Myrtaceae root material (7000 ± 150 years B.P.) from a similar study sandy mud unit at Thirroul, some 2 km north of the Bulli site. These dates indicate that sea level reached its present position between 7500 and 6500 years ago and thus record the final stages of the postglacial marine transgression on the southern coast of New South Wales.

Mammoth from Babine Lake, British Columbia

Remains of a partially articulate mammoth skeleton were exposed during stripping operations at a mining site on Babine Lake, central British Columbia. The bones lay in silty pond deposits in a bedrock depression, and were overlain by a thin layer of gravel and a thick layer of glacial till. Although no molar teeth were found, limb proportions show that the specimen was a large mammoth, like the Columbian mammoth (Mammuthus cf. M. columbi). Two radiocarbon dates of 42 900 ± 1860 yr B.P. and 43 800 ± 1830 yr B.P. on wood from the silty fossiliferous layer, and another of 34 000 ± 690 yr B.P. on mammoth bone suggest that the animal sank in sticky pond deposits and died there. Paleobotanical evidence indicates that, during this part of the Olympia Interglaciation, the vegetation near Babine Lake was similar to present shrub tundra just beyond the treeline in northern Canada.

Geophysical exploration for buried river channels in the Gulgong Goldfields N.S.W.

A detailed magnetic and gravity survey was done on portion of the Gulgong Goldfields, New South Wales. The geophysical survey was undertaken to study the geology in an area blanketed by a thick alluvial cover and in particular to investigate a buried, Tertiary river system. This system is known to be present and was once the object of active gold mining operation. The magnetic survey revealed a complex anomaly pattern containing both normal and reversed anomalies. These features were difficult to interpret due to the paucity of geological information and did not appear to be related to the channel structures. The gravity survey was successful in distinguishing the generally broad bedrock depressions associated with the gold-bearing channels. Owing to the presence of a complex regional feature, qualitative interpretation was simplified by applying a number of second derivative operators to the gravity data. Quantitative interpretation wer known ground suggested that lateral density variations within the bedrock or alluvium did not significantly contribute to the measured anomalies and that a density contrast of 0.5 gm/cc exiats between the bedrock and alluvium.

Lake Mien — An Astrobleme or a Volcano-Tectonic Structure

Lake Mien forms a depression in the acid Precambrian bedrock in southeastern Sweden. According to the first interpretation, the brecciated and lava-like rocks found in the glacial deposits south of Lake Mien were believed to be of volcanic origin and were described as rhyolites. Recehtly, a meteoritic origin has been proposed from identification of coesite and shock-wave deformation lamellae in quartz grains in the rhyolitic rocks. In the present paper some preliminary results of field investigations in the Mien area are discussed, which in some respects do not support the meteoritic theory. Furthermore, these results establish that the peculiar rocks, described earlier as rhyolites, are not typical volcanic rocks. More detailed investigations, especially of a geophysical and petrographical character, are necessary, however, to draw a final conclusion about the origin of the Mien structure.