Abstract

Many geological inferences are based on observed associations; but associations are notoriously difficult to interpret. The orthodox interpretation of the final stages of a sedimentary basin is that termination is brought about by uplift, and the sediments are then folded by lateral compression.

The evidence of petroleum geology, particularly in S.E.Asia, is that the terminal regressive sequences of many Tertiary basins accumulated while still subsiding, and that the significant deformation for petroleum accumulation occurred before uplift, and in a stress field with a component of horizontal tension.

Faults that moved while sediment accumulated must have been subsiding for the sediment to have accumulated in both blocks; and the supply of sediment must have exceeded the local capacity to accumulate it for the observed thickness differences to have developed. The association of such faults with regressive sequences is therefore logical. The long periods of normal-fault movement and the contemporaneous growth of anticlines revealed by subsurface geology, indicate that these anticlines were formed in the same stress field as the normal faults.

The evidence seems unambiguous that these anticlines were formed in a stress field with a component of horizontal tension, while the sediments were subsiding. It is therefore concluded that the significance of orogeny, or mountain building, lies in the increased supply of sediment, leading to the accumulation of a sedimentary sequence that is mechanically unstable. The initial deformation (at least) of this sequence is not due to horizontal tectonic forces, but rather to clay diapirism.