Sand Laminae Research Articles

Deltaic Sedimentation in Athabasca Tar Sands: ABSTRACT

The Athabasca tar sands of northern Alberta underlie an area greater than 26,000 sq km and contain in excess of 700 × 109 bbl of heavy oil. Excellent outcrops of the oil-impregnated Lower Cretaceous strata (McMurray and Clearwater Formations) are present in the valley of the Athabasca River and its tributaries in the vicinity of Fort McMurray. The middle and upper members of the reservoir consist of many small deltas of modest thickness (less than 30 m) built by rivers flowing north into a lake or lagoon which occupied an elongated depression formed by extensive salt removal from evaporite beds beneath the Devonian limestone that underlies the tar sands. Lithofacies associated with the changing environments of deposition during the infilling of this depres ion include successively fluvial sand; delta-front sand and silt; delta-platform sand; and silt, clay, and nearshore marine sand. Fluvial sand: This medium- to coarse-grained, well-sorted sand, composed of quartz, k-feldspar, and muscovite, is present in the basal part of the McMurray Formation where it forms lenticular sand bodies (2-7 m thick) containing sets of medium-scale (25 cm), high-angle, cross-beds. Included in these sand bodies are shale chips, mummified logs, and abundant comminuted carbon. Grain size is uniform throughout and the upper and the lower contacts are sharp. There is little mineral cement, and there is little evidence of animal activity. Delta-front sand: This fine-grained, well-sorted, sand, composed of quartz, k-feldspar, and muscovite, is present in the middle McMurray Formation as a large, low-angle (5-7°), cross-stratified unit 20-30 m thick. Individual beds 10-20 cm thick are separated by discontinuous, thin (5-100-mm) silt layers. Internal structures include micro-cross-laminations, burrows, and castings. No megafossils are present but silt beds contain an abundant microflora of pollen and spores. The lower contact is usually sharp and the upper contact is transitional. Within this unit there is a gradual and uniform increase in shaliness from base to top and mineral cement is rare. Delta-platform sand: This fine-grained sand and silt unit, composed of quartz, muscovite, and k-feldspar, is present in the upper part of the McMurray Formation forming a widespread unit, 7-10 m thick, composed of horizontally bedded silt and sand laminae. The lower contact is transitional with the underlying delta-front unit but the upper contact with the nearshore marine sand of the Clearwater Formation is sharp. Siderite and calcite-cemented beds are common within this unit which also contains lenticular beds of brackish-water mollusks. Also present in this unit is a sparse fauna of agglutinated foraminifers together with fish teeth, and a microflora of spores, pollen, and microplankton. Nearshore marine sand: This medium-grained sand is clean in places but mainly is a poorly-sorted mixture of sand, silt, and clay composed of quartz, chert, and glauconite. This sand forms a widespread marker bed, 5-10 m thick at the base of the Clearwater Formation, and is known as the Wabiskaw Member. Bedding in this unit has been virtually destroyed by animal activity and the most conspicuous internal features are burrowing structures filled with clean sand. A rich microfauna is found in this unit consisting inter alia of calcareous Foraminifera, Radiolaria, sponge spicules, and hystrichospherids. Lower and upper contacts are sharp with a uniform upward increase in the sand to matrix ratio. The distribution of oil within the tar-sands reservoir is controlled by the original porosity and permeability of the sediments. Thus the maximum amount of oil (18-20% by weight) is present in well-sorted, clean sands which are most commonly of fluvial origin. No oil is present in the poorly-sorted, argillaceous parts of the nearshore marine sand, or in the silt beds within the deltaic complex. End_of_Article - Last_Page 840------------

Turbidites in a Glacial Sequence: A Study from the Talchir Formation, Raniganj Coalfield, India

Ancient turbidites have mostly been described from flysch sequences. This paper deals with an occurrence of non-flysch turbidites from ancient glacial-lake sediments belonging to the Talchir formation (upper Carboniferous) of the Raniganj coalfield, India. The turbidites occur, punctuating a long sequence of glaciolacustrine shales and siltstones, as sandy inter-beds, usually 40-50 cm. thick, having a graded base (often sole-marked) and laminated top with slump structures and parallel, current-ripple, and convolute laminations. The glaciolacustrine sediments also contain varve-like rhythmites containing current-generated structures like current-ripple laminations and sole marks in the sand laminae. These are also believed to have been deposited by turbidity currents. A number of pebbly mudstones, interbedded with the turbidites and the rhythmites, have been interpreted as deposits made by high-density turbidity currents. Available evidence shows that the oversteepened and overloaded front of an outwash de...

PHYSICAL LIMNOLOGY AND SEDIMENTATION IN A GLACIAL LAKE

The following conclusions stem from a study of limnology and bottom sediment in Garibaldi Lake, a glacial lake in the southern Coast Mountains of British Columbia. Meltwater streams entering the lake cause overflows or underflows depending on local conditions, notably stream temperature and sediment load, temperature distribution in the lake, and apparently the topography at the stream mouth. Conditions vary with season and tune of day. Mixing of stream and lake waters at shallow (less than 35 feet) and moderate (65 to 100 feet) depths could be demonstrated; penetration of stream waters to greater depths could not be precluded. Sediment loads of the two glacial streams entering the lake are relatively low, and persistent underflows forming significant subaqueous channels probably do not occur. The glacial streams, notwithstanding their low sediment content, provide most of the sediment reaching the lake; a minor amount of debris is apparently ice-rafted from nonglacial areas. The bottom sediment consists of rock flour in which clay minerals are rare or absent. Turbidity currents are considered responsible for the occurrence of graded laminae of fine sand and coarse silt in the deep, flat-bottomed part of the lake, up to 2½ miles from the source of this sediment. Subaqueous slumps probably caused the turbidity currents. Absence of visible varves is attributed to (1) low rate of accumulation of suspended sediment, averaging a small fraction of a millimeter a year, (2) a lag of years, rather than months, between the introduction of much of the suspended sediment into the lake and its deposition on the lake floor, and (3) obliteration of any thin and inconspicuous varves by small-scale slumping on the steeper slopes of the lake basin and by the deposits from nonperiodic turbidity currents on the flat lake floor.