Ripple marked Flagstone Fm
Type Locality and Naming
Mid-Zambezi Valley, Zimbabwe. This is another highly descriptive, classic stratigraphic name retained from the early 20th-century geological mapping of the Rhodesian (Zimbabwean) Karoo Supergroup. Lower formation in Stormberg Gr in Mid-Zambezi Basin.
Synonym: Ripple-Marked Flags Formation
Lithology and Thickness
Characterized by a striking, thinly bedded ("flaggy") sequence of alternating fine-grained sandstones, micaceous siltstones, and silty mudstones. Its defining and most diagnostic feature are the ubiquitous presence of exceptionally well-preserved wave and current ripple marks on almost every bedding plane, alongside frequent desiccation cracks (mud cracks) and small-scale cross-lamination.
Relationships and Distribution
Lower contact
Typically overlies the massive, coarse-grained, conglomeratic braided-river deposits of the Escarpment grit Fm.
Upper contact
Conformably and transitionally overlain by the highly oxidized muds and calcretes of the Fine Red Marly Sst Fm.
Regional extent
Well-exposed across the Mid-Zambezi Basin in northern Zimbabwe and southern Zambia, often forming distinctive terraced slopes or "pavements" of rippled rock above the steep cliffs formed by the Escarpment grit Fm. Lower part of the Upper Karoo sequence. It generally correlates with the upper Beaufort Gr (e.g., Burgersdorp Fm) and Molteno Fm in the Main Karoo Basin of South Africa.
GeoJSON
Fossils
Age
Depositional setting
Shallow lacustrine (lake) margins, distal floodplains, and ephemeral sheet-flood environments.
Additional Information
The transition from the Escarpment Grit to the Ripple-Marked Flags is a key stratigraphic marker in the Zambezi basin, signifying the end of the aggressive post-Permian tectonic rejuvenation and a return to quiet, subsiding basin conditions.