Lower Angwa Fm. Review & Risking, page-10

With reference to the excellent paper by P. Oesterlen and B. Millsteed, which was kindly introduced to us by geoscientists on HC: S.Afr.J.Geol.,1994,97(2),205-224,“Lithostratigraphy, palaeontology, and sedimentary environments of the western Cabora Bassa Basin, Lower Zambezi Valley, Zimbabwe”, we are aware of the following:

“The Massive Sandstone Member attains a maximum thickness of about 2500 m in the west, 1700 m in the central area, and only 500 m in the north (fault contact). The Alternations Member varies even more, having a maximum thickness of 1500 m in the central area, a minimum thickness of 500 m in the north (fault contact), and only 100 m in the west, where the Massive Sandstone Member attains its maximum”.

As I mentioned in reply to VOGC last week, based on the 1994 paper by Oesterlen and Millsteed, the estimated total thickness in the Alternations Member (Upper Angwa) and Massive Sandstone Member (Lower Angwa) in the central part of the field is (1500m +1700m) = 3200m.If we include the current estimated depth to top Upper Angwa (2200m MD) at MK-2, then the approx. depth to the base of the Lower Angwa could be 5400m.

When browsing online I came across the following reference (p204) from what appears to be a textbook on structural geology entitled “Folding”.I would like to know the author of this text, but there was no reference anywhere.

Shear or passive flow folding

“Heterogeneous simple shear, in which the intensity of shear varies through the rock, can form and amplify folds”.

Perhaps the geologic structure Oesterlen and Millsteed were referring to in their 1994 paper can be explained by Shear or Passive Flow Folding as illustrated below?The screen dump below clearly indicates that the centre of theanticline is thicker than the limbs, and therefore I have sketched the M-1 andM-2 well paths to highlight the difference.

Other forms of multi-layer folding include Buckling, but the overall thickness is expected to remain constant for this type of folding.