If you look at the basin-wide geology of the Lower and Upper...

If you look at the basin-wide geology of the Lower and Upper Angwa Formation the two units are clearly distinguishable based on sand to shale ratio (and sand quality). The Lower Angwa is more sand prone then the Upper Angwa. The Upper Angwa was initially referred to as The Alternations Member and The Upper Angwa the Massive Member.......based on lithology. Usually massive sands have better reservoir quality than sequences of intercalated/interbedded sands, shales and coals. Sand grain size is usually generally coarser and clay and fines content is lower in massive sand units.