Donald R. Lowe

Sediment Gravity Flows: II Depositional Models with Special Reference to the Deposits of High-Density Turbidity Currents

ABSTRACT Four principal mechanisms of deposition are effective in the formation of sediment gravity flow deposits. Grains deposited by traction sedimentation and suspension sedimentation respond individually and accumulate directly from bed and suspended loads, respectively. Those deposited by frictional freezing and cohesive freezing interact through either frictional contact or cohesive forces, respectively, and are deposited collectively, usually by plug formation. Sediment deposition from individual sediment flows commonly involves more than one of these mechanisms acting either serially as the flow evolves or simultaneously on different grain populations. Deposition from turbidity currents is treated in terms of three dynamic grain populations: 1) clay- to medium-grained sand-sized particles that can be fully suspended as individual grains by flow turbulence, 2) coarse-grained sand to small-pebble-sized gravel that can be fully suspended in large amounts mainly in highly concentrated turbulent suspensions where grain fall velocity is substantially reduced by hindered settling, and 3) pebble- and cobble-sized clasts having concentrations greater than 10 percent to 15 percent that will be supported largely by dispersive pressure resulting from clast collisions and by buoyant lift provided by the interstitial mixture of water and finer-grained sediment. The effects of hindered settling, dispersive pressure, and matrix buoyant lift are con entration dependent, and grain populations 2 and 3 are likely to be transported in large amounts only within flows having high particle concentrations, probably in excess of 20 percent solids by volume. Low-density turbidity currents, made up largely of grains of population 1, typically show an initial period of traction sedimentation, forming Bouma (Tb) and Tc) divisions, followed by one of mixed traction and suspension sedimentation (Td), and a terminal period of fine-grained suspension sedimentation (Te). The sediment loads of high-density turbidity currents commonly include grains belonging to populations 1, 2, and 3. Consequently, deposition often occurs as a series of discrete sedimentation waves as flows decelerate and individual grain populations can no longer be maintained in transport. Each sedimentation wave tends to show increasing unsteadiness and accelerating sedimentation rate as it evolves, passing from an initial stage of traction sedimentation, to one of mixed frictional freezing and suspension sedimentation within traction carpets, to a final stage of direct suspension sedimentation. Sequences of sedimentary structure divisions representing this succession of depositional stages are here termed the ecoR1-3) sequence, representing population 3 grains, and the S1-3) sequence, representing population 2. Deposition of the high-density suspended load leaves behind a residual low-density turbidity current composed largely of population 1 grains. At their distal ends, high-density turbidity currents deposit mainly by suspension sedimentation, forming thin (S3) divisions. These (S3) divisions are the same as Bouma (Ta) and, if subsequently capped by (Tb-e) deposited by the residual low-density flows, become the basal divisions of normal turbidities. Liquefied flows deposit by direct high-density suspension sedimentation. Grain flows of sand are characterized by frictional freezing and their deposits are limited mainly to angle-of-repose slipface units. Density-modified grain flows, in which larger clasts are partially supported by matrix buoyancy, and traction carpets, in which a dense frictional grain dispersion is driven by an overlying turbulent flow, are important in the buildup of natural deposits on submarine slopes. Cohesive debris flows depost sediment mainly by cohesive freezing, commonly modified by suspension sedimentation of the largest clasts.

The influence of sediment recycling and basement composition on evolution of mudrock chemistry in the southwestern United States

This paper reports systematic changes in mudrock composition through time on a single con- tinental cmstal block. The changes reflect both sediment recycling processes and changes through time in the composition of crystalline material being added to the sedimentary system and are related to tectonic evolution as the block matures from a series of accreted arc terranes to a stable craton. The major and trace element distributions reflect different aspects of the provenance of the mudrocks in this study. Major elements record sediment recycling processes as well as changing proportions of sedi- mentary and first-cycle source rocks. With the exceptions of KzO (which tends to increase), and SiOz and A&O3 (which show no trend), most major oxides tend to decline in relative abundance in younger mud- rocks. Patterns shown by the Index of Compositional Variability ( ( Fe03 + KrO + NaaO + CaO + MgO + MnO + TiOJIAlrO,) and by K20/A1203 indicate that the major oxide trends are due to decreasing proportions of nonclay silicate minerals and a concomitant increase in the proportion of clay minerals, probably due to decreasing input of first cycle detritus coupled with recycling of sedimentary material. Excursions from progressive trends, marked by increases in MgO, K20, and CaO, reflect episodes of large- scale input of nonclay first-cycle minerals from crystalline source rocks due to large-scale basement uplift. The chemistry of low-solubility trace elements, in contrast, is not sensitive to recycling effects and reflects the composition of first-cycle input. Incompatible elements are progressively enriched relative to compatible elements in younger mudrocks, and values for chondrite normalised rare earth elements also increase. In addition, the Eu anomaly becomes systematically more negative in younger samples. These trends cMnot be explained by diagenetic or weathering processes, and, therefore, indicate that the proportion of fnc- tionated granitic first-cycle detritus being added to the sedimentary system becomes greater with time. These results confirm the importance of tectonic setting in controlling mudrock chemistry, and also demonstrate that there is a dynamic relationship between the tectonic evolution of a continental block and the composition of its sedimentary mantle.

Photosynthetic microbial mats in the 3,416-Myr-old ocean

Recent re-evaluations of the geological record of the earliest life on Earth have led to the suggestion that some of the oldest putative microfossils and carbonaceous matter were formed through abiotic hydrothermal processes. Similarly, many early Archaean (more than 3,400-Myr-old) cherts have been reinterpreted as hydrothermal deposits rather than products of normal marine sedimentary processes. Here we present the results of a field, petrographic and geochemical study testing these hypotheses for the 3,416-Myr-old Buck Reef Chert, South Africa. From sedimentary structures and distributions of sand and mud, we infer that deposition occurred in normal open shallow to deep marine environments. The siderite enrichment that we observe in deep-water sediments is consistent with a stratified early ocean. We show that most carbonaceous matter was formed by photosynthetic mats within the euphotic zone and distributed as detrital matter by waves and currents to surrounding environments. We find no evidence that hydrothermal processes had any direct role in the deposition of either the carbonaceous matter or the enclosing sediments. Instead, we conclude that photosynthetic organisms had evolved and were living in a stratified ocean supersaturated in dissolved silica 3,416 Myr ago.

High Archean climatic temperature inferred from oxygen isotope geochemistry of cherts in the 3.5 Ga Swaziland Supergroup, South Africa

New and compiled oxygen isotope data combined with the results of geological and sedimentological studies demonstrate that enclaves of synsedimentary to very early diagenetic cherts are widely preserved in the 3.5-3.2 Ga Swaziland Supergroup, Barberton greenstone belt, South Africa. The low δ 1 8 O values of these cherts indicate extremely high ocean temperatures of 55-85 °C. Previously, the large depletion in 1 8 O shown by all Barberton cherts relative to their Phanerozoic counterparts has been attributed to low 1 8 O in Archean oceans, chert formation during late diagenesis, wholesale loss of 1 8 O during alteration, and/ or regional silicification of sediments around hot springs. These alternative explanations are not compatible with the new results. Cherts in the Onverwacht Group display an isotopic stratigraphy that is inversely repeated in conglomerates in the overlying Fig Tree and Moodies Groups, demonstrating that the chert δ 8 0 O values were fixed prior to Archean uplift and erosion, which started at 3.26 Ga. The maximum δ 1 8 O value in Barberton cherts (+22‰) is lower than the minimum values (+23‰) in Phanerozoic bedded cherts, precluding late diagenesis as the explanation of the overall low δ 1 8 O values. Regional metamorphic, hydrothermal, or long-term resetting of original δ 1 8 O values is also precluded by preservation of δ 1 8 O values across different metamorphic grades and by systematic δ 1 8 O differences among interbedded chert types, stratigraphic units, and conglomerate clasts. The 7‰ δ 1 8 O variation of these Archean cherts is similar to that of Phanerozoic deep-sea cherts-formed when opal converted to microquartz during burial-but the actual Archean values are ∼10‰ lower. Marine opal was apparently converted to microquartz during burial to depths of <1 km. Cherts with δ 1 8 O < 15‰ reflect conversion during deepest burial or in local areas of enhanced geothermal gradient and/ or hydrothermal activity. Cherts with higher δ 1 8 O values formed during early diagenesis and indicate an extremely hot Archean ocean and surface environment.

Chronology of early Archaean granite-greenstone evolution in the Barberton Mountain Land, South Africa, based on precise dating by single zircon evaporation

We report precise 207Pb/206Pb single zircon evaporation ages for low-grade felsic metavolcanic rocks within the Onverwacht and Fig Tree Groups of the Barberton Greenstone Belt (BGB), South Africa, and from granitoid plutons bordering the belt. Dacitic tuffs of the Hooggenoeg Formation in the upper part of the Onverwacht Group yield ages between 3445 +/- 3 and 3416 +/- 5 Ma and contain older crustal components represented by a 3504 +/- 4 Ma old zircon xenocryst. Fig Tree dacitic tuffs and agglomerates have euhedral zircons between 3259 +/- 5 and 3225 +/- 3 Ma in age which we interpret to reflect the time of crystallization. A surprisingly complex xenocryst population in one sample documents ages from 3323 +/- 4 to 3522 +/- 4 Ma. We suspect that these xenocrysts were inherited, during the passage of the felsic melts to the surface, from various sources such as greenstones and granitoid rocks now exposed in the form of tonalite-trondhjemite plutons along the southern and western margins of the BGB, and units predating any of the exposed greenstone or intrusive rocks. Several of the granitoids along the southern margin of the belt have zircon populations with ages between 3490 and 3440 Ma. coeval with or slightly older than Onverwacht felsic volcanism, while the Kaap Valley pluton along the northwestern margin of the belt is coeval with Fig Tree dacitic volcanism. These results emphasize the comagmatic relationships between greenstone felsic volcanic units and the surrounding plutonic suites. Some of the volcanic plutonic units contain zircon xenocrysts older than any exposed rocks. These indicate the existence of still older units, possibly stratigraphically lower and older portions of the greenstone sequence itself, older granitoid intrusive rocks, or bodies of older, unrelated crustal material. Our data show that the Onverwacht and Fig Tree felsic units have distinctly different ages and therefore do not represent a single, tectonically repeated unit as proposed by others. Unlike the late Archaean Abitibi greenstone belt in Canada, which formed over about 30 Ma. exposed rocks in the BGB formed over a period of at least 220 Ma. The complex zircon populations encountered in this study imply that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones. A surprising similarity in rock types, tectonic evolution, and ages of the BGB in the Kaapvaal craton of southern Africa and greenstones in the Pilbara Block of Western Australia suggests that these two terrains may have been part of a larger crustal unit in early Archaean times.

Grain Flow and Grain Flow Deposits

ABSTRACT The term grain flow is restricted to sediment gravity flows in which a dispersion of cohesionless grains is maintained against gravity by grain dispersive pressure and in which the fluid interstitial to the grains is the same as the ambient fluid above the flow. Modified flows include those in which a dense interstitial fluid, current, or escaping pure fluid aids in maintaining the dispersion. Conclusions regarding the dynamics of grain flows have been based largely on the analysis of Bagnold (1954) of fully confined, gravity-free dispersions. Natural flows are neither fully confined nor gravity-free, and their characteristics and dynamics differ significantly from his experimental systems. Velocity equations are developed and used to analyze natural grain flows. Unmodified subaerial and subaqueous, steady and uniform flows of sand occur only on slopes at or near the angle of repose, are generally less than 5 cm thick, and cannot individually account for the formation of thick sedimentation units. On slopes inclined at less than the angle of repose, grain flows collapse and freeze; on higher slopes, they accelerate, dilate, and become increasingly influenced by fluid forces. Thick grain flows of gravel-sized debris and thick flows modified by the presence of dense, plastic mud interstitial to the clasts (debris flows), polymodal coarse-sediment size distribution (density-modified grain flows), or concurrent sediment liquefaction or fluidization (liquefied or fluidized sediment flows) can move over relatively low slopes and accumulate as thick sedimentation units.

The Characteristics and Origins of Dish and Pillar Structures

ABSTRACT Dish structure is defined by the presence of thin, subhorizontal, flat to concave-upward, argillaceous laminations in siltstone and sandstone units. It is commonly associated with vertical or nearly vertical cross-cutting columns and sheets of massive sand termed pillars. Both form commonly in sediment ranging in grain size from coarse-grained siltstone to coarse-grained, conglomeratic sandstone. In sedimentation units greater than about 0.5 m thick, dish structure is faint and neither cuts across nor is cross-cut by other sedimentary structures. In thinner units dish structures commonly cut across primary flat laminations, climbing-ripple cross-laminations, and convolute laminations. Dish and pillar structures form during the consolidation of rapidly deposited, underconsolidated or quick beds. During gradual compaction and dewatering, semi-permeable laminations act as partial barriers to upward-moving fluidized sediment-water slurries, forcing horizontal flow beneath the laminations to points where continued vertical escape is possible. As water seeps upward through the confining laminations, fine sediment, planar, and low-density grains are filtered out and concentrated in the sediment pore spaces. The resulting clay- and organic-enriched laminations are flat dishes that may be later deformed by the upward pressure of flow around their margins and central subsidence as underlying sediment and water escape. Pillars form during forceful, explosive water escape. It s suggested that the shapes of dishes and pillars within an individual bed can be related to its original water content, thickness, and grain size; to the rate and magnitude of dewatering including consideration of water entering the bed from underlying consolidating sediments; and to the types and distribution of earlier-formed sedimentary structures. Dish structures cannot be used directly to infer transport or depositional processes. Where dishes are associated with or cut across primary sedimentary structures, the latter indicate deposition from currents. The study, indicates that coarse-grained terrigenous sediments often have pronounced and complex consolidation histories. Many rapidly deposited beds undergo partial liquefaction and fluidization during consolidation but retain sufficient strength to resist wholesale downslope flowage in response to gravity.

Restricted shallow-water sedimentation of Early Archean stromatolitic and evaporitic strata of the Strelley Pool Chert, Pilbara Block, Western Australia

Abstract The 3.4 Ga-old Strelley Pool Chert is a 25-m thick sedimentary unit near the top of the predominantly volcanic Warrawoona Group in greenstone belts of the eastern Pilbara Block, Western Australia. It is here subdivided into 5 members containing 13 lithofacies. The basal Member, I, is composed of quartzose sandstone deposited in a high-energy wave- or tide-dominated shallow-water system. Overlying this are Members II and III, which make up the bulk of the formation and were deposited in a low-energy, partially restricted hypersaline basin. They record a predominantly regressive succesion of deposits including subaqueous laminite, stromatolite and evaporite; stromatolite, carbonaceous laminite, black-and-white banded chert, evaporite and intraformational detrital units deposited under intermittently to predominantly exposed conditions; and subaerially deposited windblown sand, evaporite and evaporite-solution layers. Members IV and V record the progradation of a volcaniclastic alluvial fringe. The Strelley Pool Chert represents an association of sedimentary environments directly comparable to that observed in modern, low-energy, shallow-marine carbonate-evaporite systems, such as along the Trucial Coast of the Persian Gulf, and abundantly developed in Phanerozoic carbonate platform deposits. There is no evidence, however, that uniquely identifies the environment as having been marine. Deposition may have taken place in either a large hypersaline lake or a restricted marine basin. Evidence of predominantly low energy depositional conditions and a paucity of terrigenous detritus indicate that sedimentation was dominated by orthochemical and biological processes. Silicified evaporites, including coarsely crystalline layers resembling Messinian selenite, are widespread and similar to younger evaporite deposits. They clearly indicate that evaporites were common within shallow-water Archean sequences. The presence of an assemblage of biogenic deposits, including organic laminite, stromatolites, encrusting carbonaceous mats, carbonaceous granules and oncolites, deposited under conditions ranging from fully subaqueous to nearly subaerial and locally evaporitic, points to the existence of an ecologically and probably biologically diverse microbial community 3.4 Ga ago.

Oxygen isotope geochemistry of cherts from the Onverwacht Group (3.4 billion years), Transvaal, South Africa, with implications for secular variations in the isotopic composition of cherts

δ18O values for 87 chert samples from the 3.4-b.y.-old Onverwacht Group, South Africa, range from +9.4 to +22.1‰. δ-values for cherts representing early silicified carbonates and evaporites, and possible primary precipitates range from +16 to +22‰ and are distinctly richer in18O than silicified volcaniclastic debris and cherts of problematical origin. The lower δ-values for the latter two chert types are caused by isotopic impurities such as sericite and feldspar, and/or late silicification at elevated temperature during burial. Cherts with δ-values below +16‰ are thus not likely to yield geochemical data relevant to earth surface conditions. Fine-grained chert is less than 0.7‰ depleted in18O relative to coexisting coarse drusy quartz. Because coarse quartz preserves its isotopic composition with time, the maximum amount of post-depositional lowering of the δ-values of cherts by long-term isotopic exchange with meteoric groundwaters does not exceed 0.7‰ in 3.4 b.y. In response to metamorphism the δ-values of Onverwacht cherts appear to remain unchanged or to have increased by as much as 4‰. Neither metamorphism nor long-term isotopic exchange with groundwaters can explain why Onverwacht cherts are depleted in18O relative to their Phanerozoic counterparts. Meteoric waters with a δ18O range of at least 3‰ appear to have been involved in Onverwacht chert diagenesis. δ-values for possible primary cherts or cherts representing silicified carbonates and evaporites are compatible with the depositional and diagenetic environments deduced from field and petrographic evidence. Onverwacht cherts appear to have formed with δ-values at least 8‰ lower than Phanerozoic cherts. The new Onverwacht data combined with all published oxygen isotope data for cherts suggest a secular trend similar to that initially suggested by Perry (1967) in which younger cherts are progressively enriched in18O. However, Precambrian cherts appear to be richer in18O than Perrys original samples and can be reasonably interpreted in terms of declining climatic temperatures from ∼70°C at 3.4 b.y. to present-day values, as initially suggested by Knauth and Epstein (1976). This surface temperature history is compatible with existing geological, geochemical, and paleontological evidence.

Prolonged magmatism and time constraints for sediment deposition in the early Archean Barberton greenstone belt: evidence from the Upper Onverwacht and Fig Tree groups

The single zircon evaporation, SHRIMP ion-microprobe and conventional dissolution techniques were used to determine 207Pb/206Pb and UPb ages on samples from the Upper Onverwacht and Fig Tree groups of the early Archean Barberton greenstone belt, South Africa. Zircons from dacitic rocks of the upper Hooggenoeg Formation yield ages of ∼ 3445–3452 Ma. A tuff in the basal Kromberg Formation has a mean age of 3416 ± 5 Ma. A tuffaceous band, 5 cm thick, in the uppermost Kromberg Formation contains igneous zircons with a mean age of 3334 ± 3 Ma. The 1700 m section of Kromberg Formation between these two samples is composed of basaltic lavas, minor komatiites and cherty metasediments. The overlying Mendon Formation is composed of interbedded komatiitic lavas and metasediments with a minimum thickness of 600 m. A cherty, stromatolitic metasediment 300 m above the base contains several thin ash layers with a mean zircon age of 3298 ± 3 Ma. The basal Fig Tree Group has units as old as 3259 ± 3 Ma, and upper units in the Fig Tree are as young as 3225 ± 3 Ma. Xenocrystic zircons in the Upper Onverwacht and overlying Fig Tree Group samples suggest that successive igneous units inherited zircons from underlying units and that, over several hundred million years, episodes of intermediate to felsic igneous activity took place at 20–40 Ma intervals. Structural repetition by isoclinal folding and thrust faulting are important components of late greenstone belt evolution, but should not obscure the importance of the prolonged interval of magmatic evolution represented by the thick pile of volcanic rocks observed in the Barberton greenstone belt.