Internal Deformation in Layered Evaporite Sequences: Evacuation and Diapirism Versus Contraction and Inflation

Abstract

Layered evaporite sequences (LES) consist of interbedded lithologies of variable strength ranging from weak carnallite and halite to stronger anhydrite, carbonates, siliclastics, and volcanics. The internal deformation depends largely on the mode of salt tectonics. Here, we compare and contrast the internal geometries produced during vertical subsidence of minibasins and associated passive diapirism or during lateral contraction and consequent inflation. In a 2D section, bed lengths increase as material moves from beneath minibasins into diapirs that grow in height, with the extension magnitude partly dependent on diapir width. This results in linked stretching and folding of incompetent layers, boudinage of competent layers into stringers, and rotation of stringers to steeper attitudes that is enhanced by vertically-oriented shear within the diapir. There is also a component of contraction in 3D as material converges from surrounding minibasins into the diapir, creating radial folds and curtain folds. The combination of stringer disruption and rotation results in low seismic coherence and the transparent seismic character typical of passive diapirs. In contrast, lateral shortening of an LES leads to internal contractional folding and consequent inflation of the salt. Because rocks are much stronger under compression than under tension, bedding is more likely to be maintained in more competent stratal packages. Moreover, bending stresses lead to extension and mechanical breakup in anticlines but contraction and bed coherence in synclines. The result on seismic data is a greater tendency to image preserved internal layering than in passive diapirs, with synclines observed more commonly and anticlines more likely to be disrupted and therefore transparent. Examples of both styles can be observed in numerous salt basins, but the best example is that of the Brazilian offshore. Although different areas originally had the same LES with similar internal layering, the seismic character differs greatly. In the proximal Santos Basin and the Campos Basin, for example, passive diapirs are largely transparent. However, over the São Paulo Plateau (distal Santos Basin), internal layering is prominent and characterized by gently to strongly folded strata enhanced by internal density contrasts. This is an area dominated by contraction and inflation, with complex internal deformation, in response to basinward movement of salt in the footwall of the Cabo Frio fault zone.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015