- GeoRef, Copyright 2004, American Geological Institute.
The Main Zone of the western lobe of the Bushveld Complex is easily and logically subdivided into five subzones which can be recognized in the field. Although not as dramatically layered as the Lower, Critical, and Upper Zones of the Complex, the Main Zone also displays significant layering. This paper presents a new stratigraphy which, although based on previous schemes, recognizes the significance of layered packages and has as its main concept for subdivision, the variation in relative abundance of different pyroxenes throughout the Main Zone. Subzone A is a noritic lithology which is well documented and has at the top an obvious transition to gabbronorite in subzone B. Subzone B is characterized by the occurrence of glassy euhedral crystals of primary orthopyroxene, together with minor clinopyroxene, in a homogeneous gabbronorite. This is separated from subzone C by the Zebra Unit, a layered package which is approximately 80 m thick. In subzone C, inverted pigeonite is present within relatively homogeneous gabbronorite. The C subzone is extensively exposed since the majority of the positive topographic relief in the area is of this rock type. Towards the top of this subzone is the Hexrivier Unit, comprising 80 m or so of rhythmically layered gabbronorites. Above this marker unit, the gabbronorite grades over approximately 25 m vertically into a rhythmically layered microgabbronorite which is approximately 75 m thick. This microgabbronorite marks the transition from subzone C to D. Subzone D is characterized by the reappearance of primary orthopyroxene. At the base, euhedral crystals of primary orthopyroxene up to 1 cm in size characterize a porphyritic gabbronorite facies which is interlayered with a gabbronorite facies containing needle-like clinopyroxene crystals in a matrix of plagioclase feldspar. This facies at the base of subzone D is at least 25 m thick. Subzone E is mineralogically similar to subzone C and is characterized by the presence of inverted pigeonite within the microgabbronorite and by the increasing occurrence of accessory magnetite. The textural and mineralogical difference between the five subzones is distinctive and is recognizable both in the field and in thin section. This five-fold subdivision is not confined to the western Bushveld and can also be recognized in the lithologies of the eastern Bushveld. Thus, this new stratigraphy is widespread, continuous, and easily correlated in the field.