| The Moodies Group is the oldest known, well-preserved, quartz-rich sedimentary sequence on earth. Sandstone petrography, detailed mapping, correlation of measured sections, paleocurrent and structural analysis, and zircon geochronology were used to explore provenance, depositional and tectonic setting, style of deformation, and age of deposition of the Moodies Group.; Moodies strata are divided regionally into two contrasting petrographic and stratigraphic sequences by the Inyoka Fault. The lower two-thirds of the Moodies north of the fault are characterized petrographically by a maturation trend and form a deepening- and fining-upward sequence. The dominantly marine upper third of the Moodies Group is characterized petrographically by rapidly fluctuating trends, and stratigraphically by southward-thinning alluvial conglomerates which were shed from the north. At least one of the conglomerates overlies an intraformational angular unconformity and marks the beginning of basin shortening by south-directed thrust faulting. This deformation eventually incorporated most of the BGB into a major fold-and-thrust belt. Thinner Moodies sections south of the Inyoka Fault may be correlative with the basal Moodies Group north of the fault. However, its sandstones lack feldspar and were probably deposited in one or more separate basins.; Monocrystalline quartz and K-feldspar were derived from intermediate and felsic plutonic rocks, probably located northwest of the present outcrops. All remaining grain types can be accounted for by rocks within the BGB. Metamorphic sources, such as the Ancient Gneiss Complex, did probably not contribute significantly to the Moodies Group.; Moodies Group rocks are folded tightly in a series of subparallel, plunging and overturned synclines separated by narrow, strike-parallel fault zones. Much of the sequence was buckle-folded above a detachment within altered ultramafic rocks. The tectonic transport direction was inward from the northern and southern margins of the greenstone belt.; The Moodies Group illustrates the formation and deformation of several short-lived alluvial to shallow marine sedimentary basins on unconsolidated continental crust. Their likely extensional origin and subsequent synsedimentary shortening, folding, and subhorizontal displacement attest to the dominance of horizontal tectonics in a setting that was probably characterized by interaction among small tectonic elements. |
