Neil Lundberg

Controls on the composition of fluvial sands from a tropical weathering environment: sands of the Orinoco River drainage basin, Venezuela and Colombia

Fluvial sands in the Orinoco River drainage basin fall into three main compositional groupings: (1) sands of subarkose and arkose composition, from high-relief parts of the Guayana Shield, where crystalline rocks are exposed; (2) sands of litharenite and sub-litharenite composition from the active orogenic belt at the western and northwestern margins of the drainage basin, and in those parts of the Llanos (Andean foreland basin) proximal to the mountain belt; and (3) sands of quartz-arenite composition, widespread throughout the remainder of the basin. Multicycle sands of quartz-arenite composition are produced from platform cover on parts of the elevated shield and from uplifted foreland-basin fill in the eastern Llanos. First-cycle sands of quartz-arenite composition are produced from granitic rocks on low-relief regions of the Guayana Shield and from reworked Holocene alluvium in parts of the western Llanos distal to the orogenic terranes. Erosion in the Orinoco River drainage basin may be described in terms of transport-limited and weathering-limited denudation regimes. In active orogenic terranes and in parts of the elevated shield, transport processes can remove weathered material as rapidly as it is produced by chemical weathering. Thin soils and short soil-mineral residence time result in sands that are incompletely chemically weathered and accurately reflect source-rock composition. In the orogenic terranes, subtle variations in source-rock lithology are preserved in sand composition. In contrast, in low-relief parts of the Guayana Shield and on flat erosion surfaces of the upland shield, weathering exceeds the rate at which transport processes can remove weathered material. Thick soils accumulate, soil-mineral residence time is long, and detritus is highly altered chemically. On much of the lowland Guayana Shield, upper soil layers consist of nearly pure quartz sand that erodes to produce first-cycle fluvial sand of quartz-arenite composition. Chemically weathered orogenically derived sand enters the Orinoco River on the left bank, while feldspathic shield-derived sand enters on the right bank. This geometry is responsible for the nearly total lack of longitudinal variation in sand composition along the 1,400-km length of the Orinoco River mainstem. Except in the upper 100 km of the Orinoco River mainstem, cross-channel heterogeneity in sand composition is also modest. Nevertheless, a weighted linear least-squares modeling approach suggests that sand moves down the Orinoco River mainstem in distinct pulses, perhaps corresponding to times of accelerated erosion in headwater regions. Because rivers that head in the orogenic terranes and traverse the Llanos contribute more than 99% of the sand in the lower Orinoco River mainstem, the composition of this sand is dominated by chemically weathered sands from the Llanos. The Orinoco River—the third largest river in the world—delivers first-cycle sands of quartz-arenite composition to its delta.

Cenozoic history of the Himalayan-Bengal system: Sand composition in the Bengal basin, Bangladesh

Stratigraphic sequences preserved in the Bengal basin provide detrital information that documents a significantly older history of the eastern Himalaya than that available from Ocean Drilling Program and Deep Sea Drilling Project cores recovered from the Bengal fan. The Bengal basin, formed as a result of the Himalayan collision, is located at the juncture of the Indian craton to the west, the Shillong massif and the Himalayan belt to the north, and the Indo-Burman ranges to the east. Modal analyses of Eocene and Oligocene sandstones of the Cherra, Kopili, and Barail Formations document compositions that are dominated by subangular monocrystalline quartz grains with scarce to no feldspar grains, and few lithic fragments (Cherra and Kopili: Qt 99 F 1 L 0 ; Barail: Qt 90 F 3 L 7 ; Qt—total quartz, F—feldspar, L—lithic fragments). These compositions are similar to sands derived from the Indian craton, suggesting that they underwent intense chemical weathering, likely in a source with low relief and considerable transport. Himalayan tectonism during this time was probably significantly more distant from the Bengal basin than at present. Sandstones of the Miocene Surma Group (Bhuban and Boka Bil Formations) are rich in feldspar grains, argillite, and very low-grade metamorphic lithic fragments (Bhuban: Qt 66 F 15 L 19 ; Boka Bil: Qt 57 F 23 L 20 ) relative to older sandstones, suggesting onset of uplift and erosional unroofing in the eastern Himalaya, and initiation of stream systems supplying orogenic detritus to the Bengal basin. Sands of the upper Miocene to Pliocene Tipam Group and the Pliocene–Pleistocene Dupi Tila Sandstone contain abundant argillitic and low- to medium-grade metamorphic lithic fragments and feldspar grains (Tipam: Qt 61 F 19 L 19 ; Dupi Tila: Qt 70 F 13 L 17 ), suggesting continued orogenic unroofing. These younger sands are rich in potassium feldspar (P/F=0.30, 0.20) relative to plagioclase (P)-rich Bhuban and Boka Bil sandstones (P/F=0.66 and 0.48), suggesting a granitic source, probably the Miocene leucogranites of the High Himalayan Crystalline terrane. These results document for the first time contrasts in orogenic history recorded in the Bengal system vs. western Himalayan foreland basins. Sands deposited in the Bengal basin are generally more quartzose and less lithic than those from the western Himalayan foreland basins, and pre-Miocene strata in the Bengal system show little to no evidence of orogenic activity. In part, this probably reflects west to east progression of the Himalayan collision, but it probably also results from sedimentary systems propagating southward, ahead of the advancing mountain belt as it has been consuming the remnant ocean basin trapped between the Indian craton and the Burmese block.

Forearc-basin closure and arc accretion in the submarine suture zone south of Taiwan

Abstract The obliquely propagating Taiwan collision provides an active example of an intraoceanic arc being accreted to a young rifted continental margin. The actual accretion of the exotic arc is taking place immediately south of Taiwan, in a complex area of rapid uplift and shortening between the emerging crest of the submarine accretionary prism and the extinct northernmost segment of the Luzon volcanic arc. The northern part of this region accommodates over half of the convergence between the Philippine Sea and Eurasian plates, based on recent results of triangulation and Global Positioning System studies. Assuming that nearly all the plate convergence to the south, in the region of normal intraoceanic subduction, is concentrated near the active trench, as is true in most subduction zones, this region of arc accretion is a zone across which approximately 60% of the total plate convergence, amounting to about 50 mm/yr, is being actively transferred. This transfer of slip is presumably caused by the buoyant nature of continental crust that has been subducting beneath the Taiwan orogen. This arcward transfer of plate convergence has strongly affected development of the suture between the Luzon arc and the continental margin, represented by the Taiwan mountain belt. Backthrusting of the accretionary prism in this region is accommodated on east-vergent thrust faults, which locally reach the surface and deform the entire forearc sequences, thereby building the Huatung Ridge, a distinct structural and bathymetric ridge east of the main accretionary prism (the Hengchun Ridge). The Huatung Ridge dams orogenic sediment from the emergent collision in the Southern Longitudinal Trough, a suture basin that projects directly northward to the Longitudinal Valley of eastern Taiwan. Growth strata in the Southern Longitudinal Trough document progressive uplift of the Huatung Ridge to the east, apparently along east-verging thrust systems. Seismic reflection and sidescan sonar data south of about 23°N provide no evidence of back-arc thrusting along the eastern margin of the Luzon arc, as has been hypothesized in order to transfer shortening to the Philippine Sea plate. Neither do these data show clear evidence of west-vergent thrusting of the Luzon arc over adjacent elements of the forearc, in contrast to the very active thrusting documented onland to the north, along the Longitudinal Valley of eastern Taiwan. The arcward-vergent structures in the region of arc accretion have closed the North Luzon Trough, the major forearc basin. These structures have also built the Huatung Ridge as a compressional ridge of orogenic strata that serves to broaden the accretionary prism toward the arc (eastward) and, in so doing, have formed small collisional or suture basins and redirected orogenic sedimentation patterns throughout this key area. Thus the arcward flank of the collision has evolved in a much more complicated fashion than the relatively smooth progression followed by the western, frontal slope of the submarine accretionary prism as it evolves northward to the fold-and-thrust belt exposed along strike in western Taiwan. This complexity on the arcward flank of the collision zone is likely a response of the collision complex to continued plate convergence in the face of increasing resistance, to the north, to the subduction of continental crust.

A paleo-Brahmaputra? Subsurface lithofacies analysis of Miocene deltaic sediments in the Himalayan-Bengal system, Bangladesh

Abstract The Bengal foreland basin contains a succession of up to 16+ km of dominantly deltaic deposits, eroded from the eastern Himalayas and the Indo–Burman ranges and carried by major river systems similar to the present-day Ganges and Brahmaputra. Analysis of electric logs and core descriptions acquired during oil and gas exploration in Bangladesh allows construction of lithofacies maps, which constrain depositional patterns of Miocene strata. Compilations of sand thickness and sand/shale ratio of the Miocene Surma Group show that Lower to Middle Miocene strata of the Bhuban Formation accumulated in a large, elongate trough. Sand thickness and percentage both decrease markedly away from this depocenter, which describes a large-scale bend, running initially westward from Rashidpur (northeast Bengal basin) and curving southward toward the Bengal fan. Middle to Upper Miocene strata of the Boka Bil Formation show a similar geographic trend in deposition of coarsest and thickest sediment, but the major depocenter had shifted northward relative to that of the Bhuban Formation by some 30 km, passing near Beani Bazar. These trends suggest that deltaic deposits of the Surma Group filled the Sylhet trough of the northeast Bengal basin from the east. Published seismic data from western Bangladesh show that additional large channels also contributed materials to the Bengal basin from the northwest during the Late Miocene, but these channels resulted in very little accumulation in the northwestern part of the basin, probably due to restricted subsidence of underlying continental crust. This study suggests that there was a major drainage system similar to the modern Brahmaputra River during Miocene time, which carried orogenic sediments eroded from the uplifted terranes of the eastern Himalayas and Indo–Burman ranges to the eastern Bengal delta.

Geology and tectonic evolution of a juvenile accretionary terrane along a truncated convergent margin: Synthesis of results from Leg 66 of the Deep Sea Drilling Project, southern Mexico

Drilling results from the Pacific margin of southern Mexico indicate that this region is characterized by a Neogene accretionary wedge, progressively emplaced against older, tectonically truncated continental crust. Accretion has occurred by both offscraping of sediments at the base of the trench slope and underplating of sediments at depth beneath the accretionary wedge and the continental crust. Mass-balance and incremental-uplift studies suggest that about one-third of the incoming sediment is subducted beneath the leading edge of the continental crust. Piston and drill cores indicate that the trench is sand dominated and flanked by slopes covered principally by mud. A large submarine canyon bypasses sediment past the shelf and inner trench slope. The volume of sediment bypassed to the trench and adjacent lower and outer slope equals 5 to 6 times that deposited on the shelf, upper slope, and mid-slope. The muddy inner slope is characterized by foraminiferan-free mud below the calcite compensation depth (CCD) and foraminiferan-bearing mud above the CCD. The upper slope accumulates laminated mud within the oxygen minimum zone. The shelf is covered by sand and mud. Quartzofeldspathic sand compositions in the Leg 66 area reflect sources in the crystalline basement complex exposed along the coast. Structural fabrics of Leg 66 cores from offscraped and overlying slope deposits show zones of inconsistent dip, stratal disruption, and scaly mudstone, characteristics of many melange-wedges exposed on land. Deformation transgresses the boundary between the offscraped and slope deposits, demonstrating tectonic incorporation of the slope sediments into the accretionary wedge. The rate of deformation of the slope deposits diminishes rapidly landward from the trench. Deposits overlying the continental crust show dip patterns due to mesoscopic folding, as well as local spaced cleavage and faulting, but they show no stratal disruption or scaly mudstone Oblique-slip faulting predominates between the accreted wedge and the continental crust and may reflect decoupling of these two basement types. Frozen sediment, probably bearing gas hydrate, was recovered above a bottom-simulating reflector at two sites.

Rapid Quaternary emergence, uplift, and denudation of the Coastal Range, eastern Taiwan

A thick sequence of orogenic strata exposed in the Coastal Range of eastern Taiwan documents uplift of at least 7.5 and 5.9 km at two anticlinal crests following marine deposition of the uppermost preserved beds at ∼1.0 Ma. The resulting mean uplift rates of 7.5 and 5.9 mm/yr are minimum values for localities that represent relative maxima in total uplift, and are consistent with previous studies that document general uplift of about 5 mm/yr in Taiwan. Despite rapid uplift, the Coastal Range is transected by the antecedent course of the Hsiukuluan River. Maintenance of this transverse drainage, rather than defeat and diversion to a ready alternate course to the sea provided by the range-bounding Longitudinal Valley, suggests that uplift has been steady over time and exemplifies a general balance between uplift and denudation rates in Taiwan. Denudation in the Coastal Range approximates the rapid present-day denudation documented in the much higher Central Range, due at least in part to less resistant rock types in the Coastal Range. Uplift of the Coastal Range is driven by the tectonic incorporation of the Luzon island arc into the Taiwan mountain belt (and thus into the Asian continental margin), probably over a west-dipping decollement hypothesized to have formed during eastward propagation of the eastern deformation front of the dominantly west-vergent orogen.

Evolution of the slope landward of the Middle America Trench, Nicoya Peninsula, Costa Rica

Summary The Nicoya Peninsula of Costa Rica represents an uplifted portion of the trench slope break landward of the Middle America Trench, and is composed of the Nicoya Complex and its sedimentary cover. The Nicoya Complex is upper Mesozoic oceanic crust, showing the effects of continued igneous activity and deformation probably due to the Late Cretaceous initiation of subduction in an oceanic region. The sedimentary cover reflects uplift and progressive deformation throughout the evolution of this intra-oceanic arc-trench system. Sediments of the Nicoya Complex are open-ocean deposits, comprising locally derived sedimentary breccias made up of recycled Nicoya Complex material as well as more typically pelagic radiolarian cherts, black shales and deep-water limestones. The basal unit of the sedimentary cover is the mainly hemipelagic Campanian Sabana Grande Formation, which was probably deposited on the juvenile trench slope. In the Sabana Grande Formation, basal siliceous mudstones give way up-section to the foraminifer-rich calcareous mudstones, representing the passage of the sediment surface up relative to the CCD. Interbedded conglomerates and sedimentary breccias reflect recycling of Nicoya Complex material, apparently by uplift and erosion of the forearc basement. The Sabana Grande Formation is overlain by the ?Campanian to Palaeocene Rivas and Las Palmas Formations which consist of thick, mainly volcanogenic, thin- to thick-bedded turbidites and massive and/or pebbly sandstones characteristic of mid-fan facies associations. These rocks were probably deposited in a forearc basin situated landward of a structural high. In contrast, Palaeocene turbidites which crop out to the SW and seaward of the proposed high are thin-bedded, associated with redeposited hemipelagic mudstones, and interpreted as trench slope deposits. This thin-bedded turbidite/hemipelagic mudstone unit is in turn overlain by Palaeocene or Eocene calcareous mudstones and Eocene siliceous mudstones, both of which are interpreted to represent continued deposition on the trench slope, respectively above and below the CCD. These ?Palaeocene and Eocene mudstones are overlain unconformably by Eocene to ?Quaternary shallow-water clastic and carbonate deposits, reflecting rapid Eocene tectonic uplift. The general trend of shallowing in depositional environments through time is paralleled by an overall decrease in intensity of structural deformation up-section, indicating progressive deformation in this forearc terrane.

Strike-slip faults offshore southern Taiwan: implications for the oblique arc-continent collision processes

Taiwan is the site of present-day oblique arc-continent collision between the Luzon arc of the Philippine Sea plate and the Chinese continental margin. The major structural pattern revealed from marine geophysical studies in the area offshore southern Taiwan is that of a doubly-vergent orogenic belt, bounded by significant zones of thrusting on the west and east of the submarine accretionary wedge. Due to the oblique collision process, strike-slip faults could play an important role in this convergent domain. Topographic lineaments revealed from new digital bathymetry data and seismic reflection profiles confirm the existence of three sets of strike-slip faults in the collision-subduction zone offshore southern Taiwan: the N-S-trending left-lateral strike-slip faults within the Luzon volcanic arc, the NE-SW-trending right-lateral strike-slip faults across the accretionary wedge, and the NNE-SSW-trending left-lateral strike-slip faults lie in the frontal portion of the accretionary wedge. These strike-slip faults overprint pre-existing folds and thrusts and may convert into oblique thrusts or thrusts as the forearc blocks accrete to the mountain belt. A bookshelf rotation model is used to explain the observed geometrical relationships of these strike-slip fault systems. Based on this model, the counter-clockwise rotation of the forearc blocks in the area offshore southern Taiwan could have caused extrusion of the accretionary wedge material into the forearc basin. The originally continuous forearc basin is thus deformed into several closed and separate proto-collisional basins such as the Southern Longitudinal Trough and Taitung Trough. A tectonic evolution model which emphasizes on the development of various structures at different stages of the oblique arc-continent collision for the Taiwan mountain belt is proposed.

Clay mineralogy of Pliocene-Pleistocene mudstones, eastern Taiwan: Combined effects of burial diagenesis and provenance unroofing

Pliocene-Pleistocene sedimentary rocks in the Coastal Range of eastern Taiwan were deposited in a synorogenic collisional basin that developed immediately east of the growing Taiwan fold and thrust belt during active arc-continent collision. Unmetamorphosed mudstones from this stratigraphic sequence, which were buried up to 6 km deep, contain abundant detrital illite and chlorite derived from the adjacent metamorphic orogen. A surprisingly small down-section increase in percent illite layers in mixed-layer illite/smectite provides evidence for a low paleogeothermal gradient (about 14 °C/km), possibly combined with kinetic retardation of the illitization reaction resulting from the short duration of maximum burial. Illite crystallinity increases from the base to the top of the stratigraphic sequence, opposite to the expected trend for burial metamorphism. This reverse trend parallels lithic-fragment detrital modes of sandstones from the same sequence and records progressive unroofing of the nearby prograde metamorphic terrane during collision-related uplift and deformation.

Facies belts of the Middle America Trench and forearc region, southern Mexico: Results from Leg 66 DSDP

Summary The Middle America Trench SE of Acapulco is flanked by a steep canyon-incised slope and narrow shelf, showing one of a variety of sedimentary facies patterns possible at convergent margins. Piston and drill cores from this region define eight facies belts including: (1) a pelagic facies of brown clay, (2) an outer slope mud facies, (3) a trench sand facies, (4) a foraminiferan-free facies on the lower slope, (5) a foraminiferan-bearing facies on the mid-slope, (6) a laminated mud facies on the upper slope, (7) a shelf facies of sand and mud, and (8) a canyon facies of sand and gravel. The superposition of trench and lower slope sediment during accretion results in a fining upward sequence reflecting a gradual uplift of the seafloor through the trench sediment-plume. The lower limit of the foraminiferan-bearing facies is defined by the absence of in situ calcareous foraminiferans and is controlled by the calcite compensation depth. The upper slope laminated mud facies probably reflects the depth range of the oxygen minimum zone. In the Leg 66 area sedimentation rates are high in the trench and on the outer and lower slope, decrease on the mid-slope, and increase again on the shelf. On the inner shelf, waves and currents concentrate sand which funnels through a prominent submarine canyon, bypassing the mud-dominated slope and accumulating in the trench. A terrigenous sediment-plume generated by trench turbidity flow causes accelerated sediment accumulation to about 500 m above and 40 km seaward of the trench. The volume of material transported by the trench sediment-plume is five or six times greater than that moved by the shelf sediment-plume which supplies detritus to the shelf, upper slope and mid-slope environments.