Continental Margin Deposits Research Articles

Coastal upwelling and a history of organic-rich mudstone deposition off Peru

Summary The present-day upwelling circulation off Peru, the regional pattern of organic matter in surface sediments and the stable carbon isotope characteristics of Neogene and Quaternary carbonate lithologies suggest a unique feedback mechanism in continental margin deposition and subsequent alteration after burial. In such a scenario, the high bioproductivity, the position of a poleward flowing undercurrent and the rate of subsidence of margin basins appear to be the principal variables controlling this mechanism. Transfer of organic matter from the sea surface to the sea floor is particularly efficient in the upwelling ecosystem off Peru. Preservation and burial are enhanced by high bulk sedimentation rates along the upper continental slope (between 11°–15°S) at depths where the subsurface current velocities decrease below those normally associated with the poleward flow. Burial and preservation are diminished, however, where shallow water depths promote continuous reworking of the bottom sediments by onshore flows and alongshore water movement (between 6°–10°S). The resulting sedimentary facies are distinctly different from each other in that the former process yields an organic-rich (> 5 wt % C org ) and the latter process yields a calcareous (> 15 wt % CaCo 3 ) mud facies. The bulk sediment accumulation and individual component fluxes are estimated for both portions of the margin situated between 6° and 15°S latitude and lying in < 500 m of water depth. Furthermore, the chemical environment of organic-matter decomposition in the rapidly accumulating carbonate-poor facies is dominated by microbial fermentation and methanogenesis, whereas, the muds containing lesser amounts of organic matter are dominated by microbial sulphate reduction. These differences in facies composition persist throughout the subsequent stages of compaction and diagenesis. Most prominent among these is the formation of ‘organic’ dolomites with distinctly different isotopic signatures and mineral assemblages. The original upwelling facies (i.e., organic-rich muds or calcareous muds), the extent of reworking by subsurface currents, and the subsidence history of the margin basins may be inferred from these sedimentary signatures.

The upper Palaeozoic pebbly mudstone facies of peninsular Thailand and western Malaysia — Continental margin deposits of Palaeoeurasia

The Phuket Group in Peninsular Thailand and the Singha Formation in NW Malaysia are of Devonian to Lower Permian age. These approximately 3000 m thick strata, which are part of the »SE-Asian pebbly mudstone belt« stretching from Southern Tibet to Sumatra, have been interpreted as continental margin deposits (Mitchell et al. 1970). In contrast, recent papers (Bunopas et al. 1978,Stauffer 1983) propose a glaciomarine origin. These authors follow the scenario that parts of mainland SE Asia (Shan-Thai Craton) rifted away from Gondwana during the Lower Carboniferous and collided with Eurasia during the Late Triassic after crossing the Tethys Ocean under clockwise rotation of more than 180 degrees. In this paper the arguments of these authors will be discussed and a sedimentological description will be given which shows that the pebbly mudstones are not glaciomarine sediments, but rather of continental margin origin. The investigations on Phuket and Langkawi Islands, when compared with newest results from Central and North Thailand, clearly show that the »Shan-Thai Craton« was a part of, or closely connected with Palaeoeurasia (Indosinia) during Carboniferous and Permian times (comp.Helmcke 1985). The pebbly mudstones were deposited on the southern continental margin of this continent (northern margin of Tethys).

Cenozoic Radiolarians as Indicators of Neritic versus Oceanic Conditions in Continental-Margin Deposits: U.S. Mid-Atlantic Coastal Plain

Radiolarians show potential value as indicators of neritic versus oceanic conditions in Cenozoic continental-margin deposits. Analyses of radiolarian assemblages from the lower to middle Miocene Calvert Formation (mid-Atlantic Coastal Plain) produce convincing evidence that indicates increasingly neritic environments upward through the section. The Calvert assemblage consists of44 taxa, dominated by spongy spumellarians (typical of modern shelf waters). Radiolarian diversity and the ratio of nassellarians to spumellarians decrease upward through the section. Family group distribution in the Calvert assemblage appears to differ from normal pelagic radiolarian distribution. Also, certain subsurface-dwelling radiolarian taxa found in the lower, phosphatic portion of the section may indicate upwelling during deposition of that interval.

ARCHEAN AND EARLY PROTEROZOIC SEDIMENTATION STYLES IN THE KAAPVAAL PROVINCE, SOUTH AFRICA AND PILBARA BLOCK, AUSTRALIA

Archean sediments intercalated within the lower volcanic interval in the Barberton Mountain Land and Pilbara Block accumulated in an anorogenic, shallow-oceanic environment distant from a continental influence. Overlying terrigenous sediments represent continental margin deposits: in South Africa, terrigenous influx across oceanic crust was in response to crustal shortening and uplift of gneisses to the south. Both steep and shelf-rimmed continental margins are recognized. In South Africa, sedimentation was terminated by ocean closure through the northward advance of the zone of crustal shortening. The late-Archean Pongola Supergroup is interpreted as a response 10 this closure with sedimentation taking place in a cratonic shelf basin to the south. The lower Proterozoic witwatersrand and Transvaal Supergroups likewise accumulated in cratonic shelf basins with sediment derived from the north. Terrigenous sediment was supplied by fluvial systems but the dominant mode of sedimentation in lhe pericratonic seas was tida I and took place during prolonged periods of submergence of the Kaapvaal Province. Offshelf basinal equivalents are not recognized for these cratonic sequences. Intracratonic rift basins are represented by the Ventersdorp Supergroup and Soutpansberg Group and are also developed at the base of the Hamersley basin. A third style of lower Proterozoic sedimentation is represented by the chemical sedimentary unit in the Transvaal basin. A basal ramp margin developed after initial drowning of the Kaapvaal Province and evolved into a rimmed shelf margin. ln the latter, platform tidal flat and subtidal stromatolitic assemblages pass laterally into high-energy oolitic and stromatolitic plutform-edge facies. Basinal equivalents are present and consist of non-stromatolitic, ferruginous dolomite and limestone, mudstone, chert and finely-laminated iron-formation. The Hamersley Group is a lithologic equivalent of the basinal facies. Platform fades are poorly-represented in Australia, and comprise the thin Carawine Dolomite in the eastern Pilbara Block.

Pre-Orogenic and Synorogenic Diagenesis and Anchimetamorphism in Lower Paleozoic Continental Margin Sequences of the Northern Appalachians in and Around Quebec City, Canada.

ABSTRACT Thermal maturation of organic matter in shales and illite crystallinity reveal a regional southeastward increase in grade from the late middle and late stages of diagenesis to epimetamorphism in early Paleozoic continental margin deposits of the Quebec Appalachians near Quebec City. Four diagenetic or low-grade metamorphic zones are distinguished: Zone I representing the late middle stage of diagenesis with a mean random reflectance (0%) of asphaltic pyrobitumen between 1.0 and 1.5 per cent and illite crystallinity (IC) between 5.5 and 8.5 mm (0.72 and 1.10°2) zone II (late diagenetic stage) with 0 varying between 1.5 and 2.8 per cent and IC between 3.2 and 5.5 mm (0.42 and 0.72°2) or up to 8.0 mm (1.04°2) for black shales, zone III (anchimetamorphic zone) with 0 varying between 2.8 and 3.8 per cent and IC between 1.8 and 3.2 mm (0.23 and 0.422) and zone IV (epizone) with 0 greater than 3.8 per cent and IC less than 1.8 mm (less than 0.23°2). In the northwest (zones I and IIA) reflectance and illite crystallinity increase, within individual nappes, as a function of age or depth of burial. Farther southeast (zones IIB to IV) no such dependence is observed; instead, diagenetic grade increases regionally in a southeastward direction. Diagenesis in zones I and IIA is either in part or entirely pre-orogenic and has been transported during nappe movement, because in the stack of nappes near Quebec City diagenetic grade decreases downward from the higher to the lower tectonic units. Depth of burial of Cambrian deep-water clastic rocks of the Chaudiere Nappe (highest tectonic unit) was estimated as 6 to 7 km on the basis of reflectance (1.71 to 2.30%, diagenetic zone IIA) and an assumed paleogeothermal gradient of 30°C/km. This burial depth may have been achieved entirely by syndepositional overburden. For the Middle Ordovician Quebec Promontory Nappe (lowest exposed tectonic unit) a reflectance of 1.01 to 1.15 per cent (diagenetic zone I) requires 4 to 5 km of overburden, part of which was probably provided by the higher tectonic nappes, because younger sedimentary rocks of sufficient thickness that could have once overlain these Middle Ordovician deposits are not known in the region. Diagenesis in zone IIA, therefore, was most likely formed entirely before orogenesis and nappe transport; in zone I it was probably raised after nappe emplacement. The grade of diagenesis and metamorphism reached in zones IIB to IV are interpreted as related to regional synorogenic heating in conjunction with the Taconic orogeny. Thermal maturation levels in zone I indicate that the rocks may not have passed the oil window, which is of interest for petroleum exploration in Quebec.

Geotechnical properties of surficial sediments in a mega-corridor: U.S. Atlantic continental slope, rise, and deep-sea basin

Marine geotechnical studies at The National Oceanic and Atmospheric Administration's (NOAA) Atlantic Oceanographic and Meteorological Laboratories (AOML) for the past fourteen years have resulted in a considerable amount of sediment mass physical properties data, which have provided the basis for a comparative analysis of the surficial sediments of the U.S. Atlantic continental margin and deep-sea basin in a mega-corridor. A synthesis of available mass physical properties data for the Atlantic and Pacific basins was made by Keller and Bennett (1968, 1970). An extensive coring program initiated in 1974 by NOAA along the U.S. Atlantic outer continental margin (outer shelf, slope, and upper rise) between Hydrographer and Hatteras Canyons has provided data for an initial synthesis of the sediment geotechnical properties of the continental slope (Keller et al., 1979). Recent completion of laboratory analyses of the continental rise sediment cores (Lambert et al., in press) offer an opportunity to compare these suites of data. Although the data for the Atlantic deep-sea basin cover a considerably more extensive area than the data for the Atlantic continental margin, there is now adequate geotechnical properties information to show a sharp contrast between predominantly pelagic and hemipelagic deep-sea sediments and continental margin deposits which are primarily terrigenous in origin. In concert with the fining of sediments seaward off the continental margin (Bennett et al., 1977a; Keller et al., 1979), the mean water contents are higher on the rise (95%) than on the slope (88%) and deep basin (86%). In general, average water contents are significantly higher than the liquid limits for surficial slope, rise and basin sediments. Undrained shear strengths average 5–8 kPa for slope, rise and North Atlantic basin deposits, with values for the slope and rise sediments generally higher than the Atlantic basin deposits. Basin deposits have the greatest range in shear strength, as would be expected for such an extensive and sedimentologically diverse area. Sediments associated with the outer Hudson Canyon display the lowest average water content (73%), highest wet unit weight (1.60 Mg/m 3, wet bulk density) and grain specific gravity (2.76, average grain density) as compared with continental slope, rise and deep-sea Atlantic basin deposits.

A sediment budget and an analysis of geomorphic process in the Van Duzen River basin, north coastal California, 1941–1975: Summary

The coastal river basins of the northern California Coast Ranges have the highest sediment discharge of basins of comparable size with the United States (Judson and Ritter, 1964; Brown and Ritter, 1971; Knott, 1971; Janda, 1975; Brown, 1975). The high erosion rates in this region are caused by the unique combination of tectonic setting, geology, land use, and climate. This rapidly eroding region is located adjacent to the seismically active triple junction of three crustal plates near Cape Mendocino, California; late Tertiary and Quaternary underthrusting along the continental margin has caused recent (post-mid-Miocene) uplift (Silver, 1971). The area is underlain by the Mesozoic and early Cenozoic Franciscan assemblage, which is a deformed, faulted, locally sheared, and in part metamorphosed accumulation of continental-margin deposits. These deposits consist of intermixed units of melange and more coherent sandstones. The melange consists of an intensely sheared matrix of siltstone or sandy siltstone in which are dispersed large blocks and smaller boulders of sandstone, metasandstone, greenstone, chert, blueschist, and greenschist. Before European settlement, the area was vegetated with redwood (Sequoia sempervirens) , Douglas fir (Pseudotsuga menziesii) , and interior expanses of grassland and grass-oak woodland. Since 1870, the region has undergone progressively more intensive land use. Cattle and sheep grazing on the prairies became widespread by the 18709s, and harvesting of redwood in the lower basin started in the late 18809s. Extensive harvesting of Douglas fir commenced on private lands in the middle and upper portions of the basin after World War II and continued until the mid-sixties. The region has high annual rainfall (125 to 300 cm) that occurs almost entirely between October and April. Between two and six intense rainstorms each winter furnish the streamflows that transport the majority of the annual sediment yield. An important climatic feature is the occurrence at intervals of 100 to 500 yr of storms of extreme intensity and duration that may be responsible for much of the erosion in the region. One such storm occurred in December of 1964. All of the above features, the climate, tectonic setting, geology, vegetation, and land-use history, probably contribute to the high erosion rate of the region. In order to reach some understanding of the relative contributions of these factors and of how sediment moves through the watershed in this region, a detailed analysis was undertaken of the processes and distribution of erosion and deposition in the upper 575 km 2 of the 1,111-km 2 Van Duzen River basin (Kelsey, 1977). The Van Duzen is the northernmost tributary of the Eel River in north coastal California, and flows into the Eel 22 km upstream of its mouth (Fig. 1). Field work for the study took place in 1973–1976. In addition, six sets of aerial photos covering the basin from 1941 through 1975 are available, and stream-gaging records for a U.S. Geological Survey gaging station at the downstream end of the study area near Bridgeville, California, have been available since 1951. A study of changes recognizable in the aerial photographs and interpretation of stream-gaging data for 1940–1975 and sediment discharge data for the periods 1956–1967 and 1975–1976 make possible an analysis of erosion and sedimentation for the 35-yr period, 1941–1975. Surveying records going back to 1869, plus datable trees buried by flood deposits, make it possible to extend some aspects of the record to the last century.

Sedimentologic Facies in Modern Glacial-Marine Environment: ABSTRACT

Studies of recent Antarctic glacial-marine sediments have enabled us to delineate facies associations that reflect the influence of fundamental glaciologic and glacio-isostatic processes on continental-margin deposition. Glacial-marine deposits can be broadly categorized as representing three sedimentary provinces. These are (1) the grounded shelf province--that portion of the continental shelf where sediments have been deposited by grounded ice at some time during glaciation; (2) the nongrounded shelf-upper-slope province; and (3) the middle to lower slope-rise province. Sediments in the grounded shelf province consist predominantly of orthotills deposited by grounded ice, related till-flow deposits, and paratills deposited from floating ice. Glacial erosion and deposition by subglacial streams are also important processes. Seaward of the maximum grounding line, on the nongrounded shelf and upper slope, deposition is primarily by floating ice. However, substantial sediment reworking occurs, forming coarse residual deposits and sediments enriched in fine-grained material. From a sedimentologic standpoint, distinction between the outer shelf and upper slope is problematic. The middle to lower slope and continental rise may be dissected by submarine canyons or may be nonchannelized. Gravity-flow deposits are commonplace. Laminated muds, perhaps deposited by contour currents, are also widespread. The boundary separating the two slope provinces is related to a zone of glacio-isostatically induced slumping where sediment gravity flows are generated, and perhaps to the shallowest depth at which contour currents occur. These facies associations are useful in characterizing older sequences; they have been used to interpret several ancient glacial-marine sequences. End_of_Article - Last_Page 483------------

Correlation of Continental-Margin and Deep-Sea Sequences--Neogene Examples from Pacific: ABSTRACT

The Deep Sea Drilling Project (DSDP) has produced fundamental advances in Cenozoic planktonic biostratigraphy and our understanding of the Cenozoic paleo-oceanographic and paleoclimatic history of the world ocean. These new insights and data serve to increase the precision and resolution of provincial biostratigraphies and correlations commonly applied to continental-margin sequences, and thus result in more accurate reconstructions of margin histories. DSDP-IPOD Legs 18, 19, 31, 57, and 63 included drill sites close to tectonically active continental margins surrounding the North Pacific. They provide clear examples of correlation of deep-sea and epicontinental marine deposits in Mexico, California, Japan, and Korea. In particular, DSDP Site 173 off northern California h s yielded an important lower Miocene through Pleistocene (N4-N22) reference section, demonstrating the usefulness of DSDP data for interpretation of margin biostratigraphic, sedimentologic, and tectonic events on a regional scale. Multiple siliceous and calcareous plankton zones within this sequence provide an average biostratigraphic resolution of 0.5 m.y. with paleo-oceanographically induced biofacies trends marking zones of special value for interbasin correlation across latitude. Many of the planktonic datums and biofacies trends clearly defined in the thin (320 m), but nearly complete, Neogene column at DSDP Site 173 can be readily recognized in the thick paleoenvironmentally diverse and structurally disordered, continental-margin deposits now exposed along the Pacific Coast of Nort America. These correlations provide a framework for calibrating provincial biostratigraphic units; estimating rates of sediment accumulation, subsidence, and uplift in margin sequences; and hindcasting periods of increased primary productivity, variations in the oxygen-minimum layer, and deposition of sediments favorable for hydrocarbon generation. End_of_Article - Last_Page 473------------

Correlation and Timing of Platform-Margin Megabreccia Deposition, Cow Head and Related Groups, Western Newfoundland: ABSTRACT

The allochthonous Cow Head Group of Cambrian-Ordovician age is a well-exposed but laterally discontinuous succession of continental-margin deposits. Spectacular megabreccias that punctuate the succession at discrete levels can now be correlated throughout the proximal Cow Head region to the more distal Humber Arm area, and tentatively into the metamorphic terrane of the Fleur-de-Lys, far to the east. The oldest breccia is of latest Early Cambrian age (Irishtown Formation). It is composed of shallow-water limestone and sandstone (Labrador Group) as well as Precambrian basement clasts in a quartz-sand matrix. Deposition coincides with progradation of supermature quartz arenites out to and over the edge of the shelf margin (Hawke Bay Formation). The main pulses of megabreccia deposition after the Early Cambrian produced carbonate-rich units. They occurred in the medial Middle Cambrian, near the transition between Cambrian and Ordovician time, and at the transition between the Early and Middle Ordovician. These breccias contain a mixture of clasts, predominantly from the shelf margin and upper part of the slope. The youngest carbonate megabreccia, lying above the Middle Ordovician Table Head Formation, is autochthonous; it records a massive rearrangement of the continental margin during ophiolite obduction. Normal platform-margin sediments in this succession, periplatform ooze (ribbon limestone), carbonate turbidites, breccia deposits (of ribbon-limestone clasts and some platform-margin clasts), and shales coincide with flooding of the platform. Megabreccias coincide with major changes in shelf sedimentation or regression, now apparent as formation boundaries. End_of_Article - Last_Page 474------------