Abstract

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.

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