John R. Conolly

Ice-Rafted Detritus as a Climatic Indicator in Antarctic Deep-Sea Cores

Ice-rafted detritus is readily identified in sediment cores raised from the deep ocean floor around Antarctica. A few cores have reached a depth below which no ice-rafted material is found. This depth is interpreted as indicating the establishment of earliest Pleistocene glaciation in the Southern Hemisphere. It is just below a depth where there is a change in assemblages of Radiolaria which Hays associates with the Pliocene-Pleistocene boundary. The presence of ice-rafted material throughout the upper zone in cores taken south of the Polar Front indicates continuity of glaciation in Antarctica. Further north, near 45�S in the Argentine Basin, zonation of the ice-rafted detritus can be used to delineate glacial stages of the Pleistocene.

Sedimentation in the Puerto Rico Trench

ABSTRACT Holocene, Pleistocene, Tertiary and Upper Cretaceous sediments occur in the Puerto Rico Trench area. The Holocene sediments include mainly brown pteropod and foraminiferal oozes on the slope and brown abyssal clays in the hilly parts of the lower Trench and on the Outer. Ridge. The south wall scarp consists mostly of gray-green Miocene days and scree slopes containing serpentine and limestone rubble. Soft sediments overlie Upper Cretaceous andesitic volcanics, radiolarian cherts and foraminiferal cherty marls on the faulted North Wall. Color, texture, and composition of beds in ten cores 3 to 10 m long from the floor of the Puerto Rico Trench form the basis for correlation of graded beds several millimeters to 7 m thick over distances of 200 km. There is a general decrease in grain si e of the coarsest detritus and the thickness of the basal sandy section in each graded bed down a high level abyssal plain, into and along the abyssal plain of the lower Trench floor. Thick beds of megascopically homogeneous silty clay, commonly burrowed in their uppermost 10 to 20 cm, make up most of the graded beds in the deepest part of the lower Trench floor. Variation in bottom topography and sediment properties show that the layers were deposited by turbidity currents originating near the Puerto Rico-Virgin Islands shelf and flowing through numerous canyons northwestward down a high-level abyssal plain and into the lower main trench plain on which they spread laterally. In at least two instances the turbidity currents were powerful enough to deposit 20 to 50 cm of fine sand 100 km way from and 10 m above the point which they entered the main trench floor.

Late Pleistocene and Holocene Sedimentation in the Laurentian Channel

Reddish-brown glacial-marine sediment was deposited in the Laurentian Channel during the late Pleistocene. The sediment contains calcite, quartz and feldspar grains with relic iron oxide rims, and red calcite-cemented lithic sandstone and arkosic rock fragments derived principally from Triassic (and/or Carboniferous-Permian) sediments of Appalachian Canada or from their reworked derivatives. Two 10-23-cm.-thick brick-red tills, 70-350 cm. apart and containing concentrations of the above detritus, lie within the glacial marine sediment; both tills thin and converge seaward. The pebbles, granules, and sand show little evidence of glacial abrasion and may have been derived from the adjacent shallow shelf and transported by ice which advanced over the deep waters of the Laurentian Channel. If the brick-red tills were deposited at the ice front, they are time-transgressive beds within a wedge of glacial marine sediment. A strong sub-bottom reflector correlates in some areas with the upper brick-red till and in other areas with sand beneath the lower brick-red till. Brown silty clays and clayey silts lie above this reflector and, together with overlying gray sediment, are generally more than 20 feet thick and thin seaward and toward the channel walls. The coarse fraction of the gray sediments consists chiefly of angular quartz, feldspar, and amphibole-rich granodioritic rock fragments derived from the crystalline rocks of eastern Quebec.

South Sandwich tephra in deep-sea sediments☆

Abstract Eleven cores containing volcanic detritus, taken from the bottom of the South Atlantic between the South Sandwich Islands and the Mid-Atlantic Ridge, have been examined. The mineralogical composition, texture, type of transportation and the source of the volcanic detritus are discussed. Black volcanic sand layers and olivine-rich basaltic sand occur interbedded and mixed with diatomaceous ooze which is the predominant sediment in the cores. Thick black sand layers composed of basaltic ash and deposited by turbidity currents occur in the cores in the South Sandwich Islands area. Several volcanic ash layers occur in the cores east of the South Sandwich Islands and can be traced a distance of 400 miles towards the Mid-Atlantic Ridge. Three ash layers can be correlated on the basis of refractive index. The thickness and the grain size of the tephra decrease eastward from the South Sandwich Trench for 400 miles. Beyond this area tephra is dispersed in other sediment and does not form discrete layers. The tephra which consists of a mixture of brown volcanic glass, minerals, and rock fragments is most probably derived from andesite-basaltic and basaltic eruptions in the South Sandwich Islands. A study of the diatom floras indicates that the last tephra fall occurred subsequent to the last cold period, that the middle fall occurred during the penultimate warm period and that the lower tephra fell during the penultimate cold period.

Permo-Triassic Sedimentation Patterns, Sydney Basin, Australia

Fluvial, deltaic, and marine-shelf sediments were deposited in a 100- by 200-mi, north-south oriented trough centered around Sydney, where up to 12,000 ft accumulated during the Permo-Triassic. A basin analysis was made using depositional models based on rock outcrops and closely spaced boreholes. These depositional models were used to form a best fit-of-environment correlation along three lines of section through the basin using 21 deep boreholes as control. Patterns of sedimentation can be delineated from the correlation charts at several intervals through the sequence. Lower Permian sediments were derived mainly from the north and, except for minor delta-plain facies in the nearshore area, were principally deposits of a marine shelf and delta front with an abundant fau a of thick-shelled pelecypods, brachiopods, and bryozoans. Most of these marine sandstones and siltstones contain predominantly lithic fragments but some, apparently representing subaqueous bars, are distinctly quartzose. From the Middle Permian into the Early Triassic, the sediments were deposited in a southward prograding delta and were derived mainly from the north. Two fluvial wedges formed which merged southward into a deltaic plain facing a shallow sea. Close borehole control within the fluvial system establishes channel-bar sandstones and conglomerates 50-200 ft thick grading laterally into levee and lacustrine siltstones and back-swamp coal beds. Alluvial deposits grade seaward into delta-plain sediments consisting of distributary-mouth bar sandstones 30-50 ft thick which interfi ger laterally with interdistributary bay sequences. These bay sequences commonly grade from fine to coarse upward and commonly are capped by intensely burrowed sandstones or root claystones and coal beds. Delta-front sandstones flank the delta-plain deposits and merge with mottled gray siltstones of the open shelf. Several episodes of delta outbuilding separated by periods of marine transgression can be delineated with the last most widespread progradation followed in the Triassic by an equally extensive transgression of shallow marine water. Fluviodeltaic deposits in this latest episode lack coal, and the sedimentary sequence is dominated by an orthoquartzitic barrier bar-tidal delta system locally 1,000 ft thick which grades laterally into marine red claystones and gray tidal-flat silt tones and fine sandstones.

Modern Graded Beds and Turbidity Currents: Case History: ABSTRACT

Modern turbidity-current deposits are well known from the deep sea but, in most areas, core control is insufficient to establish correlation between individual graded beds. Color, texture, and composition of beds End_Page 608------------------------------ in 10 cores, 3-10 m. thick, from the floor of the Puerto Rico trench form the basis for correlation of graded beds from several millimeters to 7 m. thick over distances of 200 miles. The largest unit is 2-7 m. thick and covers an area of approximately 4,000 square miles. Variations in bottom topography and sediment properties show that the layers were deposited by turbidity currents originating near the Puerto Rico-Virgin Islands shelf. These currents flowed through numerous canyons northwestward down a high-level abyssal plain and into the lower main trench plain where they spread laterally. In at least two places the turbidity currents were powerful enough to deposit 20-50 cm. of fine sand 60 miles away from and 30 feet higher than the place near which they entered the main trench f oor. There is a general decrease in the grain-size of the coarsest detritus as well as the thickness of the basal sandy section in each graded bed down the high-level abyssal plain, into and along the trench floor. Thick beds of homogeneous clay, commonly burrowed in the uppermost 10-20 cm. of the beds, make up most of the graded beds in the deepest part of the lower trench floor. The basal sections of most graded units consist of layers of graded or laminated sand whose modal grain-size decreases upward within each graded bed. Cross-stratified fine sand occurs mainly at the base of the graded beds at the distal margins of the lower trench floor. End_of_Article - Last_Page 609------------

Potential Oil Corridor Bisects Australian Continent: ABSTRACT

New oil discoveries, coupled with intensified exploration in the past four years, suggest that several of Australias major onshore basins can be linked to form a potential oil corridor which will span the continent. The huge Canning basin forms the western part of the corridor and lies adjacent to fields in the Amadeus and Pedirka basins in central Australia which merge with the oil provinces of the central Eromanga and Cooper basins which are linked, in turn, to the eastern Surat basin. Some narrow basement arches separate and form the only barriers to a zone which crosses the Great Inland deserts of the continent. Eventually this zone could support a network of pipelines and other facilities to provide the infrastructure required for easier economic development of remo e outback regions. Parts of the infrastructure are now being developed or already exist such as the pipelines from Moomba to Sydney and Adelaide. As new oil discoveries, such as those at Blina in the Canning basin and in Jackson in the Eromanga basin, are made, this infrastructure will grow along the potential corridor and away from it to coastal waters. The corridor owes its origin in the main part to the geometry of ancient basic tectonics and subsequent sedimentation patterns. For instance, Ordovician oil-rich sequences linked the Canning and Amadeus basins, whereas Jurassic oil reservoirs cross the remainder of the eastern half of the corridor. Potential new discoveries are predicted for sequences which range in age from upper Proterozic to Neocomian and these can be reviewed within he corridor. Devonian reef trends flank the northern Canning End_Page 440------------------------------ Figure basin while Ordovician carbonates and shales form potential fields throughout parts of the southern Canning basin. Thick sequences of interbedded clastics and marine rocks of Silurian to Permian age form additional targets in several depocenters in the Canning. Oil reservoirs of Ordovician age occur in the western Amadeus basin but new plays exist in the basal Cambrian sands of the eastern Amadeus and potentially in the northern thrust sheet belt. Oil has now been found in rocks of Permian, Triassic, Jurassic, and Cretaceous age in the Cooper-Eromanga basin. New plays exist with the extensions of the central Eromanga basin where higher heat flow and deeper burial has matured younger Jurassic sources. The more established Surat basin has further potential in Permian and Triassic rocks s does the Mesozoic in the coastal Clarence-Moreton and Sydney basins. With less than 200 wildcats being drilled every year, the potential for this corridor certainly lies in the future. End_of_Article - Last_Page 441------------

Redeposition of Pelagic Sediment by Turbidity Currents; a Common Process for Building Abyssal Plains: ABSTRACT

ABSTRACT Reworked pelagic detritus forms the bulk of graded beds from the upper 10 to 20 m of most abyssal plains. Coccoliths are by far the commonest detrital particle in many of the abyssal plains and occur with discoasters in the fine silt fraction, whereas reworked radiolaria, diatoms and planktonic foraminifera occur in the coarse silt and fine sand fractions. Piston and gravity cores from the Argo and Gascoyne abyssal plains off northwest Australia contain graded beds which consist of pelagic detritus sorted into layers of either radiolaria, diatoms or planktonic foraminifera. These layers clearly define the basal portions of many graded beds of different color shades. The source for most of the sediment must lie in the pelagic oozes of the adjacent abyssal hills and rises and not in the upper continental slope and shelf areas. There is increasing evidence that density currents commonly originate in the more immediate fan valleys and rises flanking the abyssal plains and infrequently on the upper continental slope. End_of_Record - Last_Page 506-------