Foreland basin sedimentation in the Mapepe Formation, southern-facies Fig Tree Group

Abstract

The Mapepe Formation of the Fig Tree Group in the southern part of the Barberton Greenstone Belt (BGB) includes three major petrologic suites:(1) volcaniclastic rocks, (2) terrigenous rocks, and (3) chemical or precipitative deposits. These comprise three lithofacies associations reflecting the main elements of the Mapepe deposystem: (a) a basinal association, composed of fine tuff, mud, and, locally, banded ferruginous chert, jasper, and iron formation, that accumulated under quiet subaqueous conditions; (b) a fan-delta association composed of sandstone and conglomerate deposited adjacent to uplifts; and (c) a shallow-water association, including chert, jasper, bedded barite, and terrigenous and volcaniclastic units. Terrigenous sediments in the Mapepe Formation were eroded from underlying Onverwacht Group and penecontemporaneous felsic volcanic rocks. The composition of the detritus changes up section. The lowest conglomerates lack clasts of felsic volcanic rock and vary in composition from outcrop to outcrop, reflecting derivation from local, immediately underlying greenstone sources. Higher units are regionally more uniform in composition and contain clasts representing both deeper levels of erosion and felsic volcanic sources. The absence of sediment eroded from metamorphic rocks or plutonic rocks and the uniformly low detrital quartz content of Mapepe sandstones suggest that tonalitic plutons were not widely exposed in Mapepe time.The earliest Mapepe deposystem consisted of a single large basin in southeastern parts of the BGB that accumulated iron formation and ferruginous sediments. During middle Mapepe time, this basin was divided by the nascent Onverwacht anticline. Middle and late Mapepe sedimentation occurred within paired foreland basins lying along the northwestern and southeastern margins of the BGB and separated by a structural and topographic high marking the Onverwacht anticline, which may have developed as a shared forebulge. These basins were flanked to the northwest and southeast, respectively, by uplifts, probably fold-and-thrust belts. In the southeastern basin, the lower sequence of shale, iron-formation, and jasper, deposited under quiet, deep-water conditions, is overlain by shale interbedded with coarse clastic detritus derived from uplifts to the southeast. In the northwestern basin, the lowest Mapepe strata include a basal 5-to 10-m-thick shallow-water sequence of felsic tuff, chert, jasper, and barite succeeded by 100-200 m of fine deep-water felsic tuff. The overlying 200-300 m consists of basinal

The five stable isotope compositions of Fig Tree barites: Implications on sulfur cycle in ca. 3.2Ga oceans

“…The D 17 O measurement of Archean barites was first reported in Farquhar et al (2000); there is no anomaly for a set of selected Fig (Reimer, 1980) but most deposits and the thickest units are developed in the so-called barite syncline and conglomerate quarry areas in the central part of the belt (Heinrichs and Reimer, 1977;Lowe and Knauth, 1977). Barite here occurs in the Mapepe Formation (Lowe and Byerly, 1999b) and was deposited under a range of shallow-marine conditions (Lowe and Nocita, 1999). Barite beds are interlayered with sandstone and mudstone deposited during the earliest stages of tectonic activity and uplift in the Barberton belt, tuffs representing the waning stages of volcanism, and, in many areas, beds of spherules formed by large meteorite impacts elsewhere on the Earth (Lowe et al, 1989;Lowe et al, 2003).…”

Multiple sulfur-isotope signatures in Archean sulfates and their implications for the chemistry and dynamics of the early atmosphere

“…S3 D-J). These lithologies indicate the deposition in a complex, volcanically active, coastal and fandelta association (17) controlled by the emergence and erosion of tectonically unstable highs leading to rapid changes of lithology (18). The rocks of the Mapepe Formation experienced relatively minor deformation and low-grade metamorphism under lower greenschist facies (19), which resulted in the good preservation of the original sedimentary and diagenetic structures.…”

“…The barite deposits of the Mapepe Formation overlie unconformably the deep-water carbonaceous black cherts and komatiitic volcanic units at the top of the Mendon Formation (Onverwacht Group). They consist of millimeter-scale to tens of centimeter-thick beds of barite, which are hosted by a complex clastic and cherty sequence including spherule beds, thin chert layers locally ferruginous, a variety of greenish carbonated cherts representing silicified and carbonated felsic ash and containing disseminated barite locally evolving into barite beds (17), cherty sandstones containing detrital barite and jasper grains, and chertpebble conglomerates (18) (Fig. S3 D-J).…”

Sedimentology of the ∼3.3 Ga upper Mendon Formation, Barberton Greenstone Belt, South Africa

“…reflect a major change in tectonic style -the onset of orogenesis (Lowe, 1999a;Lowe and Nocita, 1999). The Mendon Formation is the uppermost unit of the Onverwacht Group.…”