ALBERT

Description: Although lunar studies suggest that large asteroid impact rates in the inner solar system declined to their present low levels at 3.8–3.7 Ga, recent studies in greenstone belts indicate that asteroids 20 km to 70+ km in diameter were still striking the Earth as late as 3.2 Ga at rates significantly greater than the values estimated from lunar studies. We here present geologic evidence that two of these terrestrial impacts, at 3.29 Ga and 3.23 Ga, caused heating of Earth’s atmosphere, ocean-surface boiling, and evaporation of tens of meters to perhaps 100 m of seawater. Rapid ocean evaporation resulted in abrupt sea-level drops, erosion of the exposed sea floor, and precipitation of distinctive layers of laminated silica representing marine siliceous sinter. Such events would have severely affected microbial communities, especially among shallow-water and photosynthetic organisms. These large impacts profoundly affected Archean crustal development, surface environment, and biological evolution until 3.2 Ga, or even later.

Description: The production of biogenic silica has dominated the marine silica cycle since early Paleozoic time, drawing down the concentration of dissolved silica in modern seawater to a few parts per million (ppm). Prior to the biological innovation of the first silica biomineralizing organisms in late Proterozoic time, inputs of silica into Precambrian seawater were balanced by strictly chemical silica and silicate precipitation processes, although the mechanics of this abiotic marine silica cycle remain poorly understood. Cherty sedimentary rocks are abundant in Archean sequences, and many previous authors have suggested that primary precipitation of amorphous silica could have occurred in Archean seawater. The recent discovery that many pure chert layers in early Archean rocks formed as sedimentary beds of sand-sized, subspherical silica granules has provided direct evidence for primary silica deposition. Here, we provide further sedimentological and geochemical analyses of early Archean silica granules in order to gain a better understanding of the mechanisms of granule formation. Silica granules are common components of sedimentary cherts from a variety of depositional settings and water depths. The abundance and widespread distribution of silica granules in Archean rocks suggest that they represented a significant primary silica depositional mode and that most formed by precipitation in the upper part of the water column. The regular occurrence of silica granules as centimeter-scale layers within banded chert alternating with layers of black or ferruginous chert containing few granules indicates episodic granule sedimentation. Contrasting silicon isotopic compositions of granules from different depositional environments indicate that isotopic signatures were modified during early diagenesis. Looking to modern siliceous sinters for insight into silica precipitation, we suggest that silica granules may have formed via multiple stages of aggregation of silica nanospheres and microspheres. Consistent with this hypothesis, Archean ocean chemistry would have favored particle aggregation over gelling. Granule formation would have been most favorable under conditions promoting rapid silica polymerization, including high salinity and/or high concentrations of dissolved silica. Our observations suggest that granule sedimentation was often episodic, suggesting that granule formation may have also been episodic, perhaps linked to variations in these key parameters.

Description: The Barberton greenstone belt (BGB) includes eight known layers containing spherical particles (spherules) that condensed from rock vapor clouds formed by the impact of large meteorites or asteroids 3.47–3.23 Ga. Previous studies have inferred that the spherules represent bolides at least 20–70 km across. Spherule beds S1–S4 have been previously characterized in detail: we provide here the first detailed analysis of more recently discovered beds S5–S8. All eight beds are composed of the same basic compositional and textural spherule types, including nearly pure silica spherules representing nonaluminous melt precursors, nearly pure phyllosilicate spherules representing mostly mafic and ultramafic liquids, and compositionally mixed spherules. Evidence of spherule amalgamation and surface corrosion within the rock vapor clouds is developed in some beds. Bed S6, which occurs within a thick sequence of ultramafic volcanic rocks, is overlain by a tsunami layer containing zircons as old as 3811 ± 7 Ma, suggesting deep, possibly impact-related crustal uplift and erosion in distant areas. The formation of at least 8 major impact layers representing bolides 20–70 km in diameter over an interval of ~240 m.y. suggests impact rates greatly exceeding those of later geologic time, and provides direct evidence that terrestrial bombardment by large bolides did not end abruptly at 3.8 Ga, but waned gradually until 3.0 Ga or even later. The coincidence of at least four large impact layers and the initiation of BGB deformation at 3.26–3.23 Ga suggests that an impact cluster at this time may have disrupted a long-lived earlier geodynamic system and triggered the development of a contrasting, more modern plate tectonic regime.

Description: A bstract :  This paper reexamines the Pliocene Repetto and Pico formations, Ventura Basin, California, to evaluate the processes of sedimentation and interpret their environmental evolution within an elongate trough basin. Analysis of stratal architecture, lithofacies distributions, paleotransport directions, and measured sections from select stratigraphic intervals show that the two formations consist of sheet-like units, 20–50 m thick, composed of three main lithofacies associations: 1) structureless to graded gravelly sandstone, 2) thick- to medium-bedded sandstone composed principally of Bouma T a /T abc /Lowe S 3 divisions with thin mudstone layers, and 3) thin- to very thin-bedded sandstone and mudstone composed of Bouma T bcde divisions. At the bed-set level, clear trends in bed thickness cannot be recognized, but the lithofacies associations show fining-upward, coarsening-upward, and symmetric stacking patterns, reflecting disorganization and frequently shifting sand and gravel depocenters, which are common trends in braided systems. Additionally, the frequent and shallow erosional features, widespread deposition, and paucity of wedge-shaped bedding geometries and associated laterally equivalent mud-rich units suggest that channel–levee elements were not present. Due to limited lateral and upslope data, the formations may represent up to three alternative depositional environments: 1) sedimentary fill of a channel that is wider than outcrop (greater than 4–5 km wide), 2) frontal lobes that necessitate at least one upslope channel, or 3) lobes emanating from canyon mouths in a loosely confined pattern. However, the flows were not affected by the margins if there existed a wide channel, and there is no evidence for channel-like incision in outcrop or core. Therefore, these units are interpreted as an unconfined network of coarse-grained, braided lobe complexes that formed elongate sedimentary aprons at the bases of canyons feeding the trough basin. This study describes deposits that are likely found in other elongate or high-latitude basins characterized by high rates of coarse-grained or clay-poor sedimentation, and is of interest to those working to understand the range of possibility in deep-water depositional systems.

Description: In the modern silica cycle, dissolved silica is removed from seawater by the synthesis and sedimentation of silica biominerals, with additional sinks as authigenic phyllosilicates and silica cements. Fundamental questions remain, however, about the nature of the ancient silica cycle prior to the appearance of biologically mediated silica removal in Neoproterozoic time. The abundance of siliceous sedimentary rocks in Archean sequences, mainly in the form of chert, strongly indicates that abiotic silica precipitation played a significant role during Archean time. It was previously hypothesized that these cherts formed as primary marine precipitates, but substantive evidence supporting a specific mode of sedimentation was not provided. We present sedimentologic, petrographic, and geochemical evidence that some and perhaps many Archean cherts were deposited predominately as primary silica grains, here termed silica granules, that precipitated within marine waters. This mode of silica deposition appears to be unique to Archean time and provides evidence that primary silica precipitation was an important process in Archean oceans. Understanding this mechanism promises new insights into the Archean silica cycle, including chert petrogenesis, microfossil preservation potential, and Archean alkalinity budgets and silicate weathering feedback processes.

Description: The ca. 3260 Ma contact between the largely volcanic Onverwacht Group and overlying largely sedimentary Fig Tree Group in the Barberton greenstone belt, South Africa, is widely marked by chert dikes that extend downward for up to 100 m into underlying sedimentary and volcanic rocks of the Mendon Formation (Onverwacht Group). In the Barite Valley area, these dikes formed as open fractures that were filled by both precipitative fill and the downward flowage of liquefied carbonaceous sediments and ash at the top of the Mendon Formation. Spherules that formed during a large meteorite or asteroid impact event occur in a wave- and/or current-deposited unit, spherule bed S2, which widely marks the Onverwacht–Fig Tree contact, and as loose grains and masses within some chert dikes up to 50 m below the contact. Four main types of chert dikes and veins are recognized: (Type 1) irregular dikes up to 8 m wide that extend downward across as much as 100 m of stratigraphy; (Type 2) small vertical dikes, most 〈1 m wide, which are restricted to the lower half of the Mendon chert section; (Type 3) small crosscutting veins, most 〈50 cm across, filled with precipitative silica; and (Type 4) small irregular to bedding-parallel to irregular veins, mostly 〈10 cm wide, filled with translucent precipitative silica. Type 2 dikes formed first and reflect a short-lived seismic event that locally decoupled the sedimentary section at the top of the Mendon Formation from underlying volcanic rocks and opened narrow vertical tension fractures in the lower, lithified part of the sedimentary section. Later seismic events triggered formation of the larger type 1 fractures throughout the sedimentary and upper volcanic section, widespread liquefaction of soft, uppermost Mendon sediments, and flowage of the liquefied sediments and loose impact-generated spherules into the open fractures. Late-stage tsunamis everywhere eroded and reworked the spherule layer. The coincidence of crustal disruption, dike formation, spherule deposition, and tsunami activity suggests that all were related to the S2 impact or impact cluster. Crustal disruption at this time also formed local relief that provided clastic sediment to the postimpact Fig Tree Group, including a small conglomeratic fan delta in the Barite Valley area. Remobilization and further movement of debris in the subsurface continued for some time. Locally, the deposition of dense baritic sediments over soft dike materials induced remobilization of material in the dike, causing foundering of S2 and ~1–2 m of overlying baritic sediments into the dike. Spherule beds occur at the base of the Fig Tree Group over wide areas of the Barberton belt, marking the abrupt change from ~300 m.y. of predominantly anorogenic, mafic, and komatiitic volcanism of the Onverwacht Group to orogenic clastic sedimentation and associated felsic volcanism of the Fig Tree Group. This area never again returned to Onverwacht-style mafic and ultramafic volcanism but evolved ~100 m.y. later into the Kaapvaal craton. These results indicate that this major transition in crustal evolution coincided with and was perhaps triggered by major impact events ca. 3260–3240 Ma.

Description: Two major features of Earth’s geochemical evolution include (1) the extraction of continental crust from the mantle to produce a depleted upper mantle, and (2) the onset of plate tectonics and shallow melting of the upper mantle to produce oceanic crust (mid-ocean-ridge basalt, MORB). Prior studies have suggested that continental crust extraction and/or plate tectonics began prior to 4 Ga or were not initiated until as late as the Neoproterozoic. Earth’s geological record is very incomplete prior to ca. 3 Ga, and the limited terrains available for study have left the nature of the earliest tectonics and crustal compositions unresolved. Here we show that a meteor impact at 3.24 Ga excavated basaltic crust and depleted mantle material. We use high field strength elements and other immobile elements to model the composition of the target rock and meteor and compare it to analyses of spherules condensed from the vaporized meteor and impact target rock. This provides insight into otherwise unpreserved Archean crust. A simple mixture of 15% CV group carbonaceous chondrite, 65% ocean basalt, and 20% depleted MORB mantle (DMM) fully replicates the field results. A significant proportion of this basalt is MORB like, distinctly different from the tholeiitic rocks that dominate in Archean greenstone belts, and consistent with seafloor spreading. The presence of DMM implies that a significant volume of continental crust was extracted before 3.24 Ga. However, no granite or tonalite-trondhjemite-granodiorite can be incorporated into the model and replicate field results, consistent with the lack of any observed shocked zircons in any of the Paleoarchean impact layers to date.