Bedrock Cores Research Articles

Preliminary estimates of regolith generation and mobility in the Susquehanna Shale Hills Critical Zone Observatory, Pennsylvania, using meteoric 10Be

Abstract This study seeks to quantify the rate and timing of regolith generation in the Critical Zone at the Susquehanna Shale Hills Critical Zone Observatory (SSHO). Meteoric 10 Be depth profiles were determined using measurements from 30 hillslope soil and bedrock core samples in an effort to constrain 10 Be inventories. The SSHO is located in the temperate climate zone of central Pennsylvania and comprises a first-order watershed developed entirely on a Fe-rich, organic-poor, Silurian-aged shale. Two major perturbations to the landscape have occurred at SSHO in the geologically recent past, including significant and sustained periglacial activity until after the retreat of the Laurentide ice sheet (∼21 ka) and deforestation during early colonial land-use. Bulk soil samples ( n = 16) were collected at three locations along a planar hillslope on the southern ridge of the catchment, representing the ridge top, mid-slope and valley floor. Rock chip samples ( n = 14) were also collected from a 24 m deep core drilled into the northern ridge top. All meteoric 10 Be concentration profiles show a declining trend with depth, with most of the 10 Be retained in the uppermost decimeters of the soil. Meteoric 10 Be inventories are higher at the mid-slope and valley floor sample sites, at 3.71 ± 0.02 × 10 10 at/cm 2 and 3.69 ± 0.02 × 10 10 at/cm 2 , than at the ridge top site (1.90 ± 0.01 × 10 10 at/cm 2 ). The 10 Be inventory at the convex ridge top site implies a minimum residence time of ∼10.6 ka, or if erosion is steady, an erosion rate of 19.4 ± 0.2 m/My.

The Use of Multiple Techniques for Conceptualisation of Lowland Karst, a case study from County Roscommon, Ireland

This paper summarises researchcarried out in county Roscom­mon, Ireland to characterise the workings of low-lying karst, of whichlittle is known. The researchemployed a combination of five main investigative techniques, in conjunction: geomor­phological mapping, spring chemistry and discharge analyses, dye-tracing, microgravity geophysical investigations and bed­rock core drilling. The results enabled the production of a de­tailed conceptual model for the area. Surface and subsurface karst landform mapping revealed a highlevel of karstification. Clustering and alignment of recharge landforms is found to be a significant aspect of the karst. Analyses of spring chemistry and discharge data revealed characteristics of the aquifer sys­tems in operation. It was found that a significant percentage of flow is via enlarged conduits but that the smaller fractures are important for providing base flow. Water tracing experi­ments proved that water moved from highly karstified, el­evated recharge zones to springs at the periphery. Microgravity geophysical investigations, detected and located solutionally enlarged voids in the bedrock and demonstrated the impor­tance of the shallow epikarst system as well as a deeper con­duit network. Bedrock core drilling detailed the nature of the bedrock underneathkarst landforms and showed the successes and failings of the geophysical investigations. Spring catchment boundaries were then delineated using water balance equations and a combination of the information retrieved from the other methods. Using these results in combination large amounts of information were gathered leading to the production of the first conceptual model for the karst of Roscommon, whichcan be adapted and applied to IrishLowlands in general. The use of multiple, complimentary, investigative techniques in conjunc­tion greatly enhanced the accuracy and success of this project. The aim of this paper, therefore is to highlight the benefits of using many analytical techniques together.

Hydrogeology of the Potsdam Sandstone in Northern New York

The Potsdam Sandstone of Cambrian age forms a transboundary aquifer that extends across northern New York and into southern Quebec. The Potsdam Sandstone is a gently dipping sequence of arkose, subarkose, and orthoquartzite that unconformably overlies Precambrian metamorphic bedrock. The Potsdam irregularly grades upward over a thickness of 450 m from a heterogeneous feldspathic and argillaceous rock to a homogeneous, quartz-rich and matrix-poor rock. The hydrogeological framework of the Potsdam Sandstone was investigated through an analysis of records from 1,500 wells and geophysical logs from 40 wells, and through compilation of GIS coverages of bedrock and surficial geology, examination of bedrock cores, and construction of hydrogeological sections. The upper several metres of the sandstone typically is weathered and fractured and, where saturated, readily transmits groundwater. Bedding-related fractures in the sandstone commonly form sub-horizontal flow zones of relatively high transmissivity. The vertical distribution of sub-horizontal flow zones is variable; spacings of less than 10 m are common. Transmissivity of individual flow zones may be more than 100 m2/d but typically is less than 10 m2/d. High angle fractures, including joints and faults, locally provide vertical hydraulic connection between flow zones. Hydraulic head gradients in the aquifer commonly are downward; a laterally extensive series of sub-horizontal flow zones serve as drains for the groundwater flow system. Vertical hydraulic head differences between shallow and deep flow zones range from 1 m to more than 20 m. The maximum head differences are in recharge areas upgradient from the area where the Chateauguay and Chazy Rivers, and their tributaries, have cut into till and bedrock. Till overlies the sandstone in much of the study area; its thickness is generally greatest in the western part, where it may exceed 50 m. A discontinuous belt of bedrock pavements stripped of glacial drift extends across the eastern part of the study area; the largest of these is Altona Flat Rock. Most recharge to the sandstone aquifer occurs in areas of thin, discontinuous till and exposed bedrock; little recharge occurs in areas where this unit is overlain by thick till and clay. Discharge from the sandstone aquifer provides stream and river baseflow and is the source of many springs. A series of springs that are used for municipal bottled water and fish-hatchery supply discharge from 1,000 to 5,000 L/min adjacent to several tributaries east of the Chateauguay River. The major recharge areas for the Chateauguay springs are probably upgradient to the southeast, where the till cover is thin or absent.

Hydraulic Properties of Variously Weathered Granitic Bedrock in Headwater Catchments

Recent studies have emphasized the importance of bedrock in hydrologic processes occurring in headwater catchments. To understand water flow processes through variously weathered bedrock, we measured the saturated hydraulic conductivity, Ks, and water retention characteristics of weakly to highly weathered Tanakami granite and Rokko granite core samples. On the basis of these core‐scale properties, along with the core shape and in situ Ks measurements, we defined two groups of bedrock: CM class (weakly weathered) and CL to DL class (moderately to highly weathered). The CM class bedrock cores had almost no effective porosity (i.e., the amount of porosity that effectively contributes to water flow) and therefore extremely small core‐scale Ks, indicating that the matrix could be regarded as essentially impermeable. The in situ Ks was much larger than the core‐scale values, however, and the core shape showed apparent fractures, suggesting that water did flow preferentially through the fractures. The volumetric water content of the CL– to DL–class bedrock water retention curves changed little in the dry range but changed gradually in the wet range, resulting in a moderate core‐scale Ks of 10−5 to 10−3 cm s−1 The core‐scale Ks values were well explained by the parameters characterizing the water retention curve. The similarity of the in situ Ks to the core‐scale values, and the lack of fractures in the core shape, suggested that water flow could be characterized as matrix flow. The hydraulic properties of weathered granite at other sites followed the trends observed at our sites, implying wide applicability of the findings in this study to various types of weathered granite.

Production rate of cosmogenic 21Ne in quartz estimated from 10Be, 26Al, and 21Ne concentrations in slowly eroding Antarctic bedrock surfaces

We estimated the production rate of 21Ne in quartz using a set of samples from slowly eroding sandstone surfaces in the Antarctic Dry Valleys. Geologic evidence as well as cosmogenic 10Be and 26Al concentrations indicate that i) these sites have experienced millions of years of surface exposure at low erosion rates, and ii) steady erosion has been sustained long enough that surface 10Be and 26Al concentrations have reached equilibrium with the erosion rate. Under these conditions, surface cosmogenic 21Ne concentrations should be a function only of the erosion rate and the 21Ne production rate. As the erosion rate can be determined from 10Be and 26Al concentrations, this allows an estimate of the 21Ne production rate. Estimating the reference 21Ne production rate on this basis, with the assumption that all 21Ne production is by neutron spallation, yields a poor fit to measured 21Ne concentrations and a systematic residual that is correlated with the erosion rate of the sample site. The same steady-erosion assumption with a production model that includes production by deeply penetrating muons yields a good fit both to our measurements and to similar, independent, measurements from an Antarctic bedrock core. Both data sets together yield a total reference 21Ne production rate of 18.3 ± 0.4 atoms g − 1 a − 1 , of which 0.66 ± 0.10 atoms g − 1 a − 1 is due to muon interactions.

High arsenic concentrations and enriched sulfur and oxygen isotopes in a fractured-bedrock ground-water system

Ground water with high arsenic concentrations (up to 26.6 μmol L − 1 ) has sulfate enriched in 34S and 18O in the fractured-bedrock, ground-water system of the Kelly's Cove watershed, Northport, Maine, USA. The ranges of sulfur and oxygen isotope values in aqueous sulfate, δ 34S [SO 4] and δ 18O [SO 4] , at the Kelly's Cove watershed are + 3.4 to + 4.9‰ and − 2.0 to + 6.7‰, respectively. These isotope values are strikingly similar to those of the Goose River, Maine watershed which has δ 34S [SO 4] and δ 18O [SO 4] ranges of + 3.7 to + 4.6 ‰ and − 2.6 to + 7.5‰, respectively. In both systems, high arsenic concentrations occur with high δ 34S [SO 4] and δ 18O [SO 4] values, yet redox conditions and underlying rock types are quite different. The isotope values of sulfide minerals, δ 34S [min], from four bedrock cores vary over short distances and range from − 5.1 to + 7.5‰. The δ 34S [SO 4] values are controlled by the δ 34S [min] values with minor input of atmospheric SO 4. The much narrower range in δ 34S [SO 4] values than δ 34S [min] values is probably due to sufficient ground-water mixing at a scale greater than the δ 34S [min] variability. The δ 34S [SO 4] values are about 2‰ higher than the average δ 34S [min] value and fall within the range of δ 34S [min] values, indicating only minor fractionation due to bacterial reduction of SO 4. The highest δ 18O [SO 4] values were measured in the downgradient, confined, arsenic-rich ground water. High δ 18O [SO 4] values there cannot be due to aeration by atmospheric oxygen, but may arise from reoxidation of reduced SO 4 products. The enrichment factors of δ 18O in SO 4 compared to H 2O, + 7.2 to + 15.5‰, in the Kelly's Cove ground water and the negligible 34S enrichment is very similar to those derived from experimental data of anaerobic sulfide oxidation in the presence of Mn and Fe oxides. Sea level at the Kelly's Cove watershed was approximately 80 m above present sea level about 13 000 years before present, imposing reducing conditions on that area of the watershed. Sea level dropped approximately 60 m below present sea level about 11 000 years before present, allowing for possible oxidation of sulfide minerals and precipitation of arsenic in ferric oxyhydroxides during aeration of the ground-water system. Under present redox conditions, there is evidence that bacteria reduction of ferric oxyhydroxides releases arsenic. The fractionation of 18O in the SO 4 during anaerobic oxidation of sulfide in the presence of Mn and Fe oxides and subsequent release of arsenic during Mn and Fe oxide reduction may explain the relationship between high arsenic concentrations and elevated 18O [SO 4] at Kelly's Cove.

New insights into the southern margin of the Archean–Proterozoic boundary in the north-central United States based on U–Pb, Sm–Nd, and Ar–Ar geochronology

New geophysical analysis of the Precambrian basement in Minnesota–Iowa–Wisconsin indicates that an Archean–Proterozoic boundary (Spirit Lake trend) previously recognized in NW Iowa can be continued eastward into central Wisconsin and farther east as the Spirit Lake tectonic zone (SLtz). To test the age of Paleoproterozoic crust south of this structure, several subsurface samples of Precambrian basement from the north-central United States have been analyzed or re-examined using modern techniques of U–Pb, Sm–Nd, and 40Ar/ 39Ar geochronology. The results fill in a major data gap for the region and show that all U–Pb crystallization ages for samples south of the SLtz are geon 17 (1700–1800 Ma). Bedrock core samples from eastern Nebraska are ca. 1760–1800 Ma, and two samples from SE South Dakota, immediately south of the SLtz, yield ages of 1762 ± 28 Ma (Vermillion) and 1733 ± 2 Ma (Elk Point). Xenoliths from impact breccia in the Manson structure in north-central Iowa yield a similar age of ca. 1705 ± 30 Ma and metagabbro from SE Minnesota yields an age of 1760 ± 9 Ma. Farther to the northeast, zircons from Paleoproterozoic gneiss in the basement of Manitoulin Island, only a few km south of the Superior craton in Ontario, also yield a geon 17 age (1714 ± 10 Ma). Sm–Nd model ages ( T DM) for samples immediately south of the SLtz fall in the range 1.9–2.2 Ga, indicating limited involvement of Archean crust. In contrast, Sm–Nd T DM ages for samples north of the SLtz typically range from 2.5 to 3.0 Ga, for both Paleoproterozoic plutons and Archean gneisses. Ion microprobe analyses of zircons from the Elk Point and Manson samples also show the presence of geon 16 overgrowths, indicating a strong regional thermal overprint during geon 16 accretion. This is supported by mid-geon 16 hornblende 40Ar/ 39Ar ages for samples from SE South Dakota and SE Minnesota. Although no U–Pb ages are available for juvenile basement beneath the ca. 1760 Ma granite–rhyolite suite of southern Wisconsin, south of the SLtz, Sm–Nd model ages are typically ca. 1.9–2.0 Ga, consistent with basement to the rhyolites being geon 17 in age. Collectively, the data require that most, if not all, of the Paleoproterozoic crust immediately south of the SLtz formed during geon 17 and probably represents eastward continuation, from Colorado, through Nebraska, of the Yavapai crustal province in the SW United States. Penokean (geon 18) crustal rocks are limited mainly to northern and central Wisconsin, east-central Minnesota, and northern Michigan. These results also show that medium grade (>500 °C) tectonothermal effects of the subsequent geon 16 (≈Mazatzal) orogeny to the south continue into the north-central United States. Both terranes probably also continue eastward into Ontario, Canada and farther east into protolith of the Grenville Province.

New data on the geological structure of the junction between the Cape Verde Seamount and the Cape Verde Basin, central Atlantic

The regions of conjugation of continental rise with abyssal oceanic basins at the margins of the Atlantic— a transitional zone between continental and oceanic lithospheres—are still poorly studied in geological terms. In the course of expeditions conducted by the Geological Institute (Moscow), the structure of this zone was studied at the continental slope of Africa, south of the Cape Verde Islands. In this area, the continental rise widens sharply making up a near-latitudinal promontory that divides the abyssal Cape Verde Basin in the south and the Canary Basin in the north. The study area is situated in the pinchout area of the system of transform fracture zones (TFZ) located south of the Fifteen Twenty TFZ [1, 2]. The near-latitudinal linear ridges and troughs on the bottom of the Cape Verde Basin are the eastern flanks of the Vema, Doldrums, Arkhangelsky, and Vernadsky TFZs of the Mid-Atlantic Ridge. Near the continental slope of Africa, these TFZs are cut off by the WNW-trending escarpment (Fig. 1). The bathymetric survey of a local area in the deepwater Cape Verde Basin that adjoins the southern margin of the Cape Verde Seamount (Figs. 1, 3) was carried out during Cruise 22 of the R/V Akademik Nikolai Strakhov in 2000. We have established an azimuthal unconformity between near-latitudinal depressions and ridges that extend from MAR, on the one hand, and the WNW-trending transversal Cabo Verde Escarpment, on the other hand [3]. The seafloor in the studied test area is complicated by volcanic edifices and by the anomalously deep (>6000 m) Strakhov Basin trending in the NW direction discordantly relative to other structural units. The previously unknown Neva deepwater channel was also found (Fig. 3). This paper has been prepared on the basis of the data collected during Cruise 16 of the R/V Akademik Ioffe in 2004. During this cruise, the sedimentary cover was studied with continuous seismic profiling (CSP). The structure of the upper part of the sedimentary cover and the bottom topography was investigated with a Parasound acoustic profilograph along a profile between 11.52 ° N × 22.67 ° W and 10.13 ° N × 24.07 ° W (Fig. 1). The structure of the upper part of the sedimentary cover in the Neva Channel and the Strakhov Basin was studied with the same method. The bedrock samples and cores of bottom sediments were recovered in the same place. Structure of the sedimentary cover (based on CSP data). The CSP profile across the junction of continen

Bedrock cores from 89° North: Implications for the geologic framework and Neogene paleoceanography of Lomonosov Ridge and a tie to the Barents shelf

Two piston cores from the Eurasian flank of Lomonosov Ridge near lat 88.9°N, long 140°E provide the first samples of bedrock from this high-standing trans-Arctic ridge. Core 94-PC27 sampled nonmarine siltstone similar in facies and age to uppermost Triassic to lower Lower Jurassic and mid– Lower Cretaceous beds in the 4 to > 5 km Mesozoic section on Franz Josef Land, on the outer Barents shelf. A ca. 250 Ma peak in the cumulative frequency curve of detrital zircons from the siltstone, dated by U- Th-Pb analysis, suggests a source in the post-tectonic syenites of northern Taymyr and nearby islands in the Kara Sea. Textural trends reported in the literature indicate that the Lower Jurassic nonmarine strata of Franz Josef Land coarsen to the southeast; this suggests the existence of a sedimentary system in which detrital zircons could be transported from the northern Taymyr Peninsula to the outer Barents shelf near the position of core 94-PC27 prior to opening of the Eurasia Basin. Correlation of the coaly siltstone in core 94-PC27 with part of the Mesozoic section on Franz Josef Land is compatible with the strong evidence from seafloor magnetic anomalies and bathymetry that Lomonosov Ridge is a continental fragment rifted from the Barents shelf during the Cenozoic. It also suggests that Lomonosov Ridge near the North Pole is underlain by a substantial section of unmetamorphosed Mesozoic marine and nonmarine sedimentary strata. Core 94-PC29 sampled cyclical deposits containing ice-rafted debris (IRD) overlying weakly consolidated laminated olive-black anoxic Neogene siltstone and mudstone with an average total organic carbon (TOC) of 4.1 wt%. The high TOC content of the mudstone indicates that during the Neogene, prior to the introduction of IRD into the Arctic seas about 3.3 Ma (early late Pliocene), the shallow waters of the central Arctic Ocean supported significant primary photosynthetic organic production near the North Pole. These deposits also contain fine grains of siltstone that resemble the breccia-clast siltstone of core 94-PC27 and reworked Carboniferous, Cretaceous, and Tertiary palynomorphs that may have also originated in the bedrock of Lomonosov Ridge.

Sea-bed pockmarks related to fluid migration from Mesozoic bedrock strata in the Skagerrak offshore Norway

In the deepest part of the Skagerrak, the sea-floor morphology and the subsurface geology have been mapped from multibeam echo sounder data, seismic data and bedrock cores. An area of high pockmark density occurs along the shallowest part of a gentle ridge in the central Norwegian Trench, where a Middle Jurassic sandstone unit subcrops below 30–40 m of Late Weichselian sediments. In contrast, few pockmarks occur at the seabed where Lower and Upper Jurassic shales and clayey siltstones subcrop. The concentration of pockmarks above the Middle Jurassic sandstone unit is thus attributed to seepage of gas or liquids migrating up-dip from deeper parts of the sedimentary basin. In the Skagerrak, more than 90% of the observed pockmarks occur above Mesozoic sedimentary rocks, while less than 10% occur in areas of Palaeozoic metasediments or crystalline rocks. A linear concentration of pockmarks is observed above a fault that terminates at the base of the Quaternary succession, and this is therefore attributed to seepage of gas or liquids along the fault plane. The density of pockmarks is low in areas with thick successions of Quaternary sediments, indicating that the pockmarks are not related to seepage of biogenic gas from these deposits. Pockmarks in the Skagerrak are thus caused by seepage of thermogenic gas and/or other pore fluids from deeper sources, and their distribution may help to improve our understanding of the fluid system and migration regime of this part of the North Sea.

Lakefloor Geomorphology of Western Lake Erie

Bathymetry of western Lake Erie, exceeding in detail any previous bathymetry, is an early result of a project conducted jointly by the U. S. National Oceanic and Atmospheric Administration and the Canadian Hydrographic Service. This bathymetry utilizes the entire archive of historic hydrographic data assembled since 1903 in the U. S. and Canada. Details of bottom configuration for the shallow (< 12 m) western portions of Lake Erie are highlighted using a 1-meter contour interval. The islands and reefs bordering and lying within the western basin have bedrock cores which are erosional remnants of the more resistant upper Silurian and lower Devonian dolomites and limestones. Overdeepened channels between the islands have been sites of postglacial nondeposition, and probably erosion, due to intense wind-driven water circulation through these restricted passages. The Pelee-Lorain Ridge is interpreted as a late Wisconsinan end moraine upon which sand deposits have been concentrated. The Point Pelee Ridge is apparently a morainic ridge, capped by sand deposits transported in part by longshore drift converging on Point Pelee. A fan-shaped feature with a surface depth of 10–15 m, which extends to the east of Point Pelee Ridge, may be a former shoreline delta of the Detroit River. This fan-shaped feature may have formed following opening of the Port Huron outlet about 4000 yr bp when the new River was actively eroding its channel and bringing a heavy load of sediment into Lake Erie—although subaqueous deposition of fan sediments is a possible alternate explanation. Sculptured drift-like features interpreted as relict spits extend northwestward and southeastward from the bedrock reefs in the western basin off Locust Point. Channels on the floor of the western Lake Erie basin underlying the main shipping lanes have been excavated by propeller wash where ship traffic increases speed, resulting in resuspension of bottom sediments. Dumpsites for dredge spoils excavated from channels are expressed in the bathymetry by a distinctive hummocky pattern in two areas.

Environmental Controls on Groundwater Chemistry in an Offshore Island Aquifer: Fiesta Key, Florida

A field study was undertaken on the Florida Bay side of Fiesta Key, Florida, to identify the chemical characteristics of a previously unexplored offshore groundwater system and to define the critical parameters affecting groundwater movement and interaction with sediment pore fluids and bedrock. Emphasis was placed on the upper 2 meters of bedrock, where groundwater recharge and discharge potentials are maximized, along a 100 meter transect extending from the island margin. Bedrock cores were used to describe Pleistocene depositional textures, and were sampled at discrete depths to determine the extent of water-rock interaction. Piezometers installed into each core hole were used to monitor surface and ground water tide levels, and for the systematic collection of water samples for a large suite of chemical determinations. Aqueous chemical data indicate that these groundwaters are marine in origin, anoxic, and moderately hypersaline (S = 36–40). Exchange of bedrock pore fluids with overlying Bay waters is restricted by a layer of Holocene sediment and a discontinuous soilstone crust formed at the modern bedrock surface. Groundwater chemistry near the sediment/bedrock interface is marked by elevated concentrations of total alkalinity and Ca2+, and by significant Mg2+ depletion. These waters likely acquired their unusual chemistry by mixing between deeper groundwaters and overlying, early diagenetically altered, sediment porewaters. High alkalinity and calcium concentrations presumably result from the combination of the effects of aerobic metabolism, carbonate dissolution, and sulfate reduction. Mg-depletion most likely resulted from the precipitation of Mg-calcite. These unusual chemistries disappear by 2 m depth in the groundwater system, where Ca2+ and Mg2+ concentrations are similar to those expected for seawater under slightly hypersaline conditions. The Pleistocene bedrock contains low Mg, Sr, F, and P concentrations relative to the overlying unconsolidated Holocene carbonate sediments. This is consistent with the diagenetic recrystallization processes that the bedrock has undergone. Hydraulic conditions favor the net recharge of Florida Bay seawater to the groundwater system, but there are insufficient tide data to identify cyclical water exchange rates or groundwater flow patterns.

Recent drumlins, flutes and lineations at Vestari-Hagafellsjökull, Iceland

AbstractIn the foreland of the Vestari-Hagafellsjökull glacier, Langajökull, central Iceland, three scales of subglacial bed forms are found: drumlins, flutes and lineations. The smaller bed forms are superimposed upon the larger. These three bed forms probably formed as a result of the presence of a rigid suhglacial core producing a lee-side low-pressure area into which sediment flowed and collected. The cores of the drumlins are tumuli (lava blisters), whilst the cores of the flutes and lineations were large clasts.Differences between the three scales of bed forms are that: (a) the flutes and lineations are far more mobile features as their cores could move within the deforming layer, whilst the drumlin bedrock cores were fixed; (b) the lineations formed in association with a clast pavement.The change in scale of the bed forms may he related to changes in thickness of the deforming layer and size of the core. At this site, smaller bed forms were superimposed upon larger bed forms and these may have formed as the deforming layer progressively thinned in association with glacial retreat.

Recent drumlins, flutes and lineations at Vestari-Hagafellsjökull, Iceland

AbstractIn the foreland of the Vestari-Hagafellsjökull glacier, Langajökull, central Iceland, three scales of subglacial bed forms are found: drumlins, flutes and lineations. The smaller bed forms are superimposed upon the larger. These three bed forms probably formed as a result of the presence of a rigid suhglacial core producing a lee-side low-pressure area into which sediment flowed and collected. The cores of the drumlins are tumuli (lava blisters), whilst the cores of the flutes and lineations were large clasts.Differences between the three scales of bed forms are that: (a) the flutes and lineations are far more mobile features as their cores could move within the deforming layer, whilst the drumlin bedrock cores were fixed; (b) the lineations formed in association with a clast pavement.The change in scale of the bed forms may he related to changes in thickness of the deforming layer and size of the core. At this site, smaller bed forms were superimposed upon larger bed forms and these may have formed as the deforming layer progressively thinned in association with glacial retreat.

Reinterpretation of a portion of the Munising moraine in northern Michigan

Field relationships between topography and surficial sediments indicate that the 25-km-long segment of the Munising moraine south of Grand Marais in Michigan9s northern peninsula is not composed of till as suggested or implied by others but consists of a series of heads of outwash and related stagnation landforms. Topographic and limited subsurface data support the interpretation that mean drift thickness in the segment is at least 10 to 20 m. These relationships show that the feature does not have a bedrock core and is not a classical end moraine. Instead, deglaciation was characterized by stagnation-zone retreat with glaciofluvial deposition at three successive ice-marginal positions defined by closely spaced morphosequences.

Paint Rock and Southwest Paint Rock Fields, Concho County, Texas: Strawn Analogs of Modern Island Carbonate Facies of Ambergris Cay, Belize: ABSTRACT

Lower Strawn (Desmoinesian Goen Limestone) reservoirs at Paint Rock and Southwest Paint Rock fields are a complex of carbonate and associated facies interpreted as having been deposited in various environments on and around large, emergent islands on shallow carbonate shelves. The origin and geometries of the component lithofacies in these fields, and their reservoir diagenetic histories, are similar to those presently accumulating on Ambergris Cay, a linear island complex on the northern shelf of Belize. Paint Rock field originated as a narrow, elongate Chaetetes reef trend that formed the foundation on which the overlying island facies were deposited. As on Ambergris Cay, these reef limestones developed extensive porosity during postdepositional subaerial exposure due to meteoric leaching. In contrast, Southwest Paint Rock field is cored by older island deposits rather than reef limestones. With ensuing stillstand or subsequent sea level rise, beach grainstones were deposited along the windward and leeward margins of the foundation highs in these fields. Tight lagoonal micrites and coals (peat-swamp facies) comprise the inner island facies, and are locally associated with porous supratidal dolomites. These island complexes are transected locally by tidal channels that are filled with nonporous micrites. Repeated sea level fluctuations during the history ofmore » these fields resulted in a characteristic cyclic stratigraphy of stacked island facies and reservoirs. The reservoirs in the field are developed in the bedrock or older island cores, as well as in the overlying beach facies and supratidal dolomites. These fields are mappable as linear stratigraphic traps with low-relief closure, and are readily identified by subsurface geologic and facies analyses. Similar shelf island-type fields analogous to these strawn and Holocene Belizean examples are found throughout the Midland basin and Eastern shelf.« less

Review of Surficial Geology and Engineering Hazards in the Canadian Offshore

Surficial geological mapping and hydrographic charting is most complete in the Atlantic Provinces offshore including the Gulf of St. Lawrence; secondarily, the British Columbia offshore and the Labrador Shelf and Slope are fairly well mapped and charted. However, the entire Arctic offshore has the poorest geological and hydrographic coverage of all Canadian marine waters, with the exceptions of Hudson Bay, Beaufort Sea-Amundsen Gulf area and Lancaster Sound.&#x0D; &#x0D; For safe marine operations, environmental information is required on winds, storms, waves, currents, tides and sea ice. This background is complemented by a seabed study designed to obtain grab and dredge samples of sediment or rock; to obtain sediment cores with the vibracorer, and gravity and piston corers; to obtain bedrock cores with the use of the diamond drill; to obtain bathymetric maps by means of sonic surveys in which echograms reveal the topographic profile of the sea-floor, and the side scan sonar give the area! view; to obtain details of bedrock features such as bedding, folds, faults, as well as distinguishing erosional and constructional topographic features all for the purposes of geological mapping; and to carry out visual surveys remotely or directly by means of submersible equipment and vehicles. All these background data and survey results can have direct application to seabed engineering problems.&#x0D; &#x0D; Potential hazards related to surficial geology in the Canadian offshore affect engineering operations such as drilling, dredging, dumping coastal works, foundations and anchorages, cable lays, and pipelines. These hazards are grouped into five main categories: (I) erosion and sedimentation, (2) Morphology, (S) movement, (4) ground ice, and (5) sediments. A breakdown is given as follows: ice-scour grooves and ridges, current scour and fill, buried river channels, sand waves, megaflutes, sand bars, valleys, fjords, canyons, troughs, thermokarst, pingos, mud diapirs, poakmarks, scarps, terraces, hummocky terrain, slump features, bedding dislocations, faulting, earthquakes, ice layers, ice-bonded sediments, thermal degradation, unstable substrate, hydrocarbons, organic matter, clathrates, hard (sand, boulders, bedrock) substrate, and moraine.&#x0D; &#x0D; The geologist must provide information and advice on the natural framework, and processes operating in the area; the engineer must appreciate these facts. But the engineer may be unable to re-locate his operational site; therefore, the geologist must present the information in its most complete and truest perspective in order to ensure a secure and successful operation.

The geology of the central Bristol Channel and the Lundy area, South Western Approaches, British Isles

A brief report is made of the main results of shallow seismic reflection, sonar and coring surveys carried out in the central Bristol Channel, between Morte Point and Porlock Bay, and the Lundy area. Lithological and palaeontological examination of 250 bedrock cores, from over 800 coring stations, has enabled a fairly detailed Jurassic succession to be identified. Hettangian to Kimmeridgian rocks probably have a total thickness of about 2170 m. The Jurassic is underlain by about 300 m of Keuper Marl and Rhaetic Beds. Keuper Marl rests unconformably on the Devonian along the south side of the Bristol Channel syncline. Late Palaeozoic shales and sandstones outcrop between Hartland Point and Lundy, and north of the island. Soft sandstones, possibly representing the Permian, overlie the Late Devonian/Early Carboniferous north of Lundy. Late Pleistocene, glacial, pebbly clays in the northern part of the Bristol Channel contain derived Triassic and Early Jurassic rock fragments and fauna. Holocene silts and peats, occurring in the depressions on the Pleistocene sediments in mid-Channel, contain Quaternary estuarine microfaunas and freshwater microfloras. Reworked Late Cretaceous and Tertiary foraminifera also present in these Holocene silts may have been derived from submarine outcrops in the Celtic Sea.

Tertiary basalts of western Davis Strait: bedrock core samples and geophysical data

Short bedrock cores of basalt were recovered at two localities on the Baffin Island shelf, 33 and 89 km southeast of Cape Dyer. The volcanic rocks underlying these sites have a surface extent of some 8000 km2 as outlined by seismic reflection and magnetic anomaly profiles. Similar rocks are inferred to occur at two smaller offshore areas south of the main area. The offshore occurrences are both more continuous and much larger than the onshore basalt areas of eastern Baffin Island.The core samples appear to have been cut from single flows consisting of fine-grained microporphyritic basalts with olivine as the principal phenocryst phase. Although having distinct differences from one another in terms of texture and degree of alteration, the samples from the three drill stations bear similarities to the Baffin Island basalts that suggest a close petrogenetic relationship may exist between the onshore and offshore basalts. However, in contrast to the subaqueously erupted volcanic breccias of onshore Baffin Island and West Greenland the offshore samples contain little evidence of glass, suggesting the possibility that the latter may have been erupted in a subaerial environment.

A KINEMATIC DISCUSSION OF THE FORMATION OF GLACIAL FLUTINGS

ALTHOUGH MANY STUDIES have been made of so-called ice-moulded landforms, drumlins, drumlinoids, and flutings, their mode of genesis remains unsolved. The form of drumlins is quite distinctive; a quotation from Alden (1905) used by Chorley (1959) clearly sets down their salient features: “We may regard as a typical drumlin a hill of glacial drift which approximates the form of a segment of an elongated ovoid, of which the widest part of the basal outline and the highest point of the crest are not more distant from the stoss-end than one-third the length of the major axis and whose major axis is oriented parallel to the direction of movement of the glacier which formed it.” With extension of the length of the characteristic drumlin a gradation occurs into extensive linear features known as flutings. The longitudinal profile of flutings differs from that of drumlins in that the maximum crest elevation is maintained for large distances. However, flutings show the same parallelism with ice-flow direction and general streamlined form as drumlins. In this paper flutings refer to both the ridges and the adjacent grooves that make up the total landform. Generally this paper will be confined to larger-scale flutings as described by Smith (1948), Gravenor and Meneley (1958), Lemke (1958), and Freschauf (1971). The large-scale features range in scale from two to three metres high and several kilometres in length. Such flutings appear to be confined to North America. Smaller-scale features have been described from a variety of recently deglaciated areas (Hoppe and Schytt 1953; McPherson and Gardner 1969; Baranowski 1970), but these are not analogous with the large-scale features. Drumlins and flutings clearly differ in form, and it is pertinent to consider other properties which may provide information on their mode of origin. A wide variety of internal composition has been noted. Till is most commonly observed, but stratified drift is often found in both drumlins and flutings. The influence of bedrock may also be important; drumlins with bedrock cores have been widely described (Chamberlin 1883; Crosby 1934), and flutings in Alberta, Saskatchewan and the Northwest Territories, Canada, may also be developed in crystalline bedrock (Smith 1948). Long-axis orientation of stones in drumlins has been shown to parallel drumlin axes approximately (Hoppe 1951; Donner and West 1955; Wright, 1957). Hill (1971) shows similar results but emphasizes a significant scatter in mean directions obtained from different samples from a single drumlin. Andrews and King (1968) show increasing deviation between stone and drumlin orientations in samples taken from successively higher positions in a drumlin section. However, micro-fabric analysis on samples from glacial flutings (Gravenor and Meneley 1958) gave orientations of silt and sand particles parallel to the fluting crest to a depth of three metres. Below three metres particle long axes were found to deviate from the fluting crest. Some authors have proposed that there is a relationship between the occurrence of drumlins