Patrick G. Eriksson

Precambrian clastic sedimentation systems

Abstract The unique and evolving nature of the Precambrian geological environment in many ways was responsible for significant differences between Precambrian clastic sedimentary deposits and their Phanerozoic-modern equivalents. Some form of plate tectonics, with rapid microplate collisions and concomitant volcanic activity, is inferred to have led to the formation of greenstone belts. Explosive volcanism promoted common gravity-flow deposits within terrestrial greenstone settings, with braided alluvial, wave/storm-related and tidal coastline sediments also being preserved. Late Archaean accretion of greenstone terranes led to emergence of proto-cratons, where cratonic and rift sedimentary assemblages developed, and these became widespread in the Proterozoic as cratonic plates stabilised. Carbonate deposition was restricted by the paucity of stable Archaean terranes. An Early Precambrian atmosphere characterised by greenhouse gases, including CO2, in conjunction with a faster rotation of the Earth and reduced albedo, provide a solution to the faint young Sun paradox. As emergent continental crust developed, volcanic additions of CO2 became balanced by withdrawal due to weathering and a developing Palaeoproterozoic microbial biomass. The reduction in CO2, and the photosynthetic production of O2, led to aerobic conditions probably being achieved by about 2 Ga. Oceanic growth was allied to atmospheric development, with approximately 90% of current ocean volume being reached by about 4 Ga. Warm Archaean and warm, moist Palaeoproterozoic palaeoclimates appear to have become more arid after about 2.3 Ga. The 2.4–2.3 Ga Huronian glaciation event was probably related to continental growth, supercontinent assembly and weathering-related CO2 reduction. Despite many analogous features among both Precambrian and younger sedimentary deposits, there appear to be major differences as well. Two pertinent examples are rare unequivocal aeolian deposits prior to about 1.8 Ga and an apparent scarcity of Precambrian foreshore deposits, particularly those related to barrier island systems. The significance of these differences is hard to evaluate, particularly with the reduced palaeoenvironmental resolution because of the absence of invertebrate and plant fossils within Precambrian successions. The latter factor also poses difficulties for the discrimination of Precambrian lacustrine and shallow marine deposits. The temporal distribution of aeolian deposits probably reflects a number of possible factors, including few exposed late Archaean–Palaeoproterozoic cratonic areas, extensive pre-vegetative fluvial systems, Precambrian supercontinents and a different atmosphere. Alternatively, the scarcity of aeolian deposits prior to 1.8 Ga may merely reflect non-recognition or non-preservation. Precambrian shallow marine environments may have been subjected to more uniform circulation systems than those interpreted from the Phanerozoic-modern rock record, and Precambrian shelves probably were broad with gentle seaward slopes, in contrast to the narrow, steep shelves mostly observed in present settings. Poorly confined Precambrian tidal channels formed sheet sandstones, easily confused with fluvial or offshore sand sheets. Epeiric seas were possibly more prevalent in the Precambrian, but active tectonism as proto-continents emerged and amalgamated to form early supercontinents, in conjunction with a lack of sufficient chronological data in the rock record, make it difficult to resolve the relative importance of eustatic and tectonic influences in forming epeiric embayments and seaways. Other differences in Precambrian palaeoenvironments are more easily reconstructed. Ancient delta plain channels were probably braided, and much thicker preserved delta successions in the Precambrian are compatible with the inferred more active tectonic conditions. Pre-vegetational alluvial channel systems were almost certainly braided as well. Common fluvial quartz arenites are ascribed to differences in weathering processes, which probably changed significantly through the Precambrian, or to sediment recycling. Although Precambrian glacigenic environments were probably the least different from younger equivalents, their genesis appears to reflect a complex interplay of factors unique to the Precambrian Earth. These include emergence and amalgamation of proto-continents, the early CO2-rich atmosphere, the development of stromatolitic carbonate platforms, early weathering, faster rotation of the Earth and the possible role of changes in the inclination of the Earths axis.

Muddy Roll-up Structures in Siliciclastic Interdune Beds of the c. 1.8 Ga Waterberg Group, South Africa

Abstract Concentrically rolled-up silty mudrock laminae, 1–2 mm thick, are found in the uppermost of four, 40–110 cm thick interdune beds within a thick aeolian succession of the c. 1.8 Ga Makgabeng Formation, Waterberg Group, South Africa. These curved laminae are analogous to previously described “roll-ups,” biogenic structures generally ascribed to soft-sediment deformation or desiccation of microbial mats overlying either carbonate or siliciclastic sediments, within shallow- to deep-marine paleoenvironments. The Makgabeng roll-ups are thought to reflect desiccation of a microbial mat, followed by resedimentation of cohesive, discrete, curled mat fragments. Their alignment is considered to be the result of an extreme precipitation event. The significance of the South African example of roll-ups is that they appear to have formed within a fully terrestrial paleoenvironment, in one of the oldest known Precambrian deserts. Thus, they represent the oldest evidence for microbial colonisation of a terrestrial setting.

Architectural elements from Lower Proterozoic braid-delta and high-energy tidal flat deposits in the Magaliesberg Formation, Transvaal Supergroup, South Africa

Abstract Three architectural elements are identified in the Lower Proterozoic Magaliesberg Formation (Pretoria Group, Transvaal Supergroup) of the Kaapvaal craton, South Africa: (1) medium- to coarse-grained sandstone sheets; (2) fine- to medium-grained sandstone sheets; and (3) mudrock elements. Both sandstone sheet elements are characterised by horizontal lamination and planar cross-bedding, with lesser trough cross-bedding, channel-fills and wave ripples, as well as minor desiccated mudrock partings, double-crested and flat-topped ripples. Due to the local unimodal palaeocurrent patterns in the medium- to coarse-grained sandstone sheets, they are interpreted as ephemeral braid-delta deposits, which were subjected to minor marine reworking. The predominantly bimodal to polymodal palaeocurrent trends in the fine- to medium-grained sandstone sheets are inferred to reflect high-energy macrotidal processes and more complete reworking of braid-delta sands. The suspension deposits of mudrocks point to either braid-delta channel abandonment, or uppermost tidal flat sedimentation. The depositional model comprises ephemeral braid-delta systems which debouched into a high-energy peritidal environment, around the margins of a shallow epeiric sea on the Kaapvaal craton. Braid-delta and tidal channel dynamics are inferred to have been similar. Fine material in the Magaliesberg Formation peritidal complexes indicates that extensive aeolian removal of clay does not seem applicable to this example of the early Proterozoic.

Late Archaean superplume events: a Kaapvaal–Pilbara perspective

Abstract The 2714–2709 Ma Ventersdorp Supergroup overlies Mesoarchaean basement rocks and sedimentary strata of the Neoarchaean Witwatersrand Supergroup. The latter basin was inverted by tectonic shortening and suffered the loss of up to 1.5 km of stratigraphy prior to deposition of the Ventersdorp volcanics. Thermal uplift and fluvial incision prior to the basal Klipriviersberg Group flood basalts appear to have been limited, but this could also reflect a hot dry palaeoclimate acting on a peneplained plateau. Rapid ascent of ponded magma beneath thinned sub-Witwatersrand lithosphere, transported laterally from a mantle plume starting head possibly situated marginally to the Kaapvaal craton is inferred for this unit of up to 2 km of predominantly tholeiitic basalts with subordinate, basal komatiites. Crustal extension related to ascent of the ponded magma followed, leading to the formation of a set of graben and half-graben basins, in which immature clastic sedimentary, and felsic to mafic lavas and pyroclastics of the Platberg Group were laid down. The Platberg basins show no evidence for reactivation of pre-existing crustal structures. The Fortescue Group of the Pilbara craton has an analogous lower flood basaltic succession, followed by graben-fills similar to those of the Platberg Group. Differences in the Fortescue include evidence for significant thermal uplift prior to the onset of volcanism, subaqueous basalts in the south of the Pilbara craton, evidence for two episodes of flood basaltic volcanism, possibly related to two plumes at c. 2765 and 2715 Ma, and graben basins aligned along existing cratonic structures. Both Kaapvaal and Pilbara flood basalts and graben-related sedimentary-volcanic deposits are thought to have been part of a c. 2.7 Ga global superplume event. The plume inferred for the Fortescue Group flood basalts was probably related to rifting and the breakup of a plate larger than the preserved Pilbara craton. Uppermost Ventersdorp units (Bothaville Formation terrestrial clastic and Allanridge Formation tholeiitic rocks) suggest a combination of thermal subsidence, allied to continued plume (minor komatiites) and graben basin influences. In the Kaapvaal craton, the Transvaal Supergroup lies unconformably above the Ventersdorp. Basal “protobasinal” successions reflect discrete fault-bounded basin-fills, analogous to those of the Platberg Group; however, it is inferred that the former depositories were related to craton marginal plate tectonic influences, specifically the c. 2.6 Ga Limpopo orogeny. Thin fluvial sheet sandstones of the Black Reef Formation unconformably succeed the protobasinal rocks and reflect the transition to an epeiric drowning of much of the Kaapvaal craton. A shallow shelf carbonate-banded iron formation platform succession (Chuniespoort-Ghaap Groups) developed in two sub-basins on the Kaapvaal craton. They are mirrored by the approximately coeval Hamersley chemical epeiric sediments on the Pilbara craton, and both Kaapvaal and Pilbara transgressive successions are related here to a possible second, c. 2.5 Ga superplume event, which raised sea levels globally. Evidence for the younger superplume event is less clear than for the c. 2.7 Ga event.

Evidence for the transition to an oxygen-rich atmosphere during the evolution of red beds in the lower proterozoic sequences of southern Africa

Abstract Strata > 2.5 to ⩽ 1.8 Ga in southern Africa appear to span the transition from a non-oxidizing atmosphere to one characterized by free oxygen. Red beds in 2.3 to 2.1 Ga unconformity-bounded sequences have matrices of stained clay minerals and are spatially associated with volcanic rocks, ironstones, or banded iron formations, which evidently were sources of iron for the matrices. However, red beds in ⩽ 1.9 Ga unconformity-bounded sequences not only matrix-staining but also hematitic coatings on grains; these beds also have far fewer interstratified ferruginous lithologies. Because red beds with stained clay matrices, banded iron formations, pyritic (Witwatersrand-type) conglomerates, and iron-depleted paleosols all occur in the ∼ 2.2 Ga Pretoria Group, formation of these lithologies may not have been time-dependent until atmospheric oxygen approached 10−2 atm. By 1.9 Ga atmospheric oxygen was ⩾ 10−2 atm, causing lateritic weathering and red beds with hematitic coatings on grains.

Assessment of chemical and mineralogical characteristics of airborne dust in the Sistan region, Iran

Windblown transport and deposition of dust is widely recognized as an important physical and chemical concern to climate, human health and ecosystems. Sistan is a region located in southeast Iran with extensive wind erosion, severe desertification and intense dust storms, which cause adverse effects in regional air quality and human health. To mitigate the impact of these phenomena, it is vital to ascertain the physical and chemical characteristics of airborne and soil dust. This paper examines for the first time, the mineralogical and chemical properties of dust over Sistan by collecting aerosol samples at two stations established close to a dry-bed lake dust source region, from August 2009 to August 2010. Furthermore, soil samples were collected from topsoil (0-5 cm depth) at several locations in the dry-bed Hamoun lakes and downwind areas. These data were analyzed to investigate the chemical and mineralogical characteristics of dust, relevance of inferred sources and contributions to air pollution. X-ray Diffraction (XRD) analysis of airborne and soil dust samples shows that the dust mineralogy is dominated mainly by quartz (30-40%), calcite (18-23%), muscovite (10-17%), plagioclase (9-12%), chlorite (~6%) and enstatite (~3%), with minor components of dolomite, microcline, halite and gypsum. X-ray Fluorescence (XRF) analyses of all the samples indicate that the most important oxide compositions of the airborne and soil dust are SiO(2), CaO, Al(2)O(3), Na(2)O, MgO and Fe(2)O(3), exhibiting similar percentages for both stations and soil samples. Estimates of Enrichment Factors (EFs) for all studied elements show that all of them have very low EF values, suggesting natural origin from local materials. The results suggest that a common dust source region can be inferred, which is the eroded sedimentary environment in the extensive Hamoun dry lakes lying to the north of Sistan.

Advanced Early Jurassic Termite (Insecta: Isoptera) Nests: Evidence From the Clarens Formation in the Tuli Basin, Southern Africa

Abstract Sandstone pillars in the Lower Jurassic eolian strata of the Clarens Formation are concentrated in clusters, with up to four pillars within 25 m2 in two localities in the Tuli Basin of northern South Africa and southern Zimbabwe. The pillars are generally vertical, have a preserved height of up to 3.3 m, and are elliptical in plan view. Pillars are grouped into two styles of architecture: those with oriented elliptical shapes and side buttresses, and those less well oriented with a smooth outer wall, internal open spaces, and vertical shafts cutting the pillar. The long axes of the elliptical pillars are generally oriented to the north. Northwards-oriented side buttresses also are associated with some of the pillars. The internal architecture of the pillars is characterized by intense bioturbation with two different burrowing styles. Type 1 burrows are composed of a network of randomly oriented, anastomosing sandstone-filled tubes, 0.3 to 0.8 cm in diameter. Type 2 burrows are rare, north-south oriented, and have a smaller diameter. Other associated features are back-filled tubes, open, vertical shafts, and open spaces between the interior and exterior of the pillars. The pillars are interpreted as fossilized termite nests. Type 1 burrows are interpreted as termite passageways within the nest. Type 2 burrows may be related to invading ants. Back-filled burrows may be a result of either beetle predation on resident termites or backfilling by termites themselves. The strong north-south orientations are comparable with modern-day nest architecture of magnetic termites in northern Australia, where nest-orientation is related to cooling. The orientations and features reported here are interpreted to be modified for the high latitudes proposed for the Lower Jurassic Clarens desert. Complex nest architecture preserved in the Clarens Formation suggests that advanced eusocial behavior and ability to construct large nests had appeared in African termites by the Early Jurassic.

Contourites associated with pelagic mudrocks and distal delta-fed turbidites in the Lower Proterozoic Timeball Hill Formation epeiric basin (Transvaal Supergroup), South Africa

Abstract Five genetic facies associations/architectural elements are recognised for the epeiric sea deposits preserved in the Early Proterozoic Timeball Hill Formation, South Africa. Basal carbonaceous mudrocks, interpreted as anoxic suspension deposits, grade up into sheet-like, laminated, graded mudrocks and succeeding sheets of laminated and cross-laminated siltstones and fine-grained sandstones. The latter two architectural elements are compatible with the Te, Td and Tc subdivisions of low-density turbidity current systems. Thin interbeds of stromatolitic carbonate within these first three facies associations support photic water depths up to about 100 m. Laterally extensive sheets of mature, cross-bedded sandstone disconformably overlie the turbidite deposits, and are ascribed to lower tidal flat processes. Interbedded lenticular, immature sandstones and mudrocks comprise the fifth architectural element, and are interpreted as medial to upper tidal flat sediments. Small lenses of coarse siltstone–very fine-grained sandstone, analogous to modern continental rise contourite deposits, occur within the suspension and distal turbidite sediments, and also form local wedges of inferred contourites at the transition from suspension to lowermost turbidite deposits. Blanketing and progressive shallowing of the floor of the Timeball Hill basin by basal suspension deposits greatly reduced wave action, thereby promoting preservation of low-density turbidity current deposits across the basin under stillstand or highstand conditions. A lowstand tidal flat facies tract laid down widespread sandy deposits of the medial Klapperkop Member within the formation. Salinity gradients and contemporaneous cold periglacial water masses were probably responsible for formation of the inferred contourites. The combination of the depositional systems interpreted for the Timeball Hill Formation may provide a provisional model for Early Proterozoic epeiric basin settings.

Sequence stratigraphy of the Precambrian Rooihoogte-Timeball Hill rift succession, Transvaal Basin, South Africa

Third-order sequence stratigraphic analysis is performed on the Rooihoogte–Timeball Hill second-order rift succession of the Paleoproterozoic Transvaal Basin, South Africa. This provides a case study for systems tract and sequence development during a time of glacio-eustatic fall, when accommodation was generated by subsidence related to syn-rift and post-rift tectonic processes. Two third-order depositional sequences have been identified, separated by a basin-wide subaerial unconformity. The lower third-order sequence includes the complete succession of lowstand, transgressive, and highstand systems tracts (LST, TST, and HST), whereas the upper third-order sequence only preserves lowstand and transgressive systems tracts. This indicates that the fall in base level associated with the upper second-order boundary of the Rooihoogte–Timeball Hill sequence was of higher magnitude relative to the third-order subaerial unconformity, which is in agreement with the principles of boundary hierarchy based on the magnitude of base-level changes. The position of the lower boundary of the Rooihoogte–Timeball Hill second-order sequence has been revised from the base of the chert breccias to the contact between the breccias and the overlying chert conglomerates. This is because a major tilting event occurred between the deposition of the two facies, which are genetically unrelated, and which are separated by a subaerial unconformity. The lithostratigraphic contact between the Rooihoogte and Timeball Hill formations is interpreted as a diachronous transgressive surface of erosion. In this interpretation, the Polo Ground Member of the Rooihoogte Formation may be coeval with the basal black shales of the Timeball Hill Formation, the two facies (fluvial and marine, respectively) forming together a transgressive systems tract. D 2002 Elsevier Science B.V. All rights reserved.

Mesoarchaean-Palaeoproterozoic stratigraphic record of the Singhbhum crustal province, eastern India: a synthesis

Abstract The Mesoarchaean–Palaeoproterozoic stratigraphic record of the Singhbhum crustal province, eastern India, implies sedimentation and volcanism in a changing tectonic scenario, and thus assumes immense geological significance. Although efforts have been made by many researchers in the past several decades to summarize various geological aspects of the Singhbhum crustal province, a critical synthesis of various stratigraphic issues was long overdue. The present contribution is an updated critical synthesis of the Mesoarchaean–Palaeoproterozoic stratigraphic record of the Singhbhum crustal province. We have pointed out the problematic stratigraphic issues of the Singhbhum crustal province that deserve careful scrutiny in order to gain better insights into the mode of stratigraphic sequence building.