Abstract

Past structural geological models for the Samarinda Anticlinorium have included gravity driven slumping, shale diapirism and thrusting related to regional tectonic events. New geological cross-sections utilising LASMO acquired data, over structures in the Runtu Block, have been constructed from both surface geological measurements and 1993–1994 vintage seismic data. Relatively rigid deltaic and shelf sediments have been deformed into box-folds, above disharmonically folded shale-rich pro-delta and bathyal sediments. Detachment of these structures appears to be at the top of, or within, over-pressured shales at the base of Lower Miocene deltaic packages.

Gravity data, acquired along with seismic data, suggests semi-regional uplifts of the underlying over-pressured strata which would have been initially deposited as a flat or gently inclined unit. Basement configuration is not visible on seismic, but gravity and aeromagnetic data show that it is deep throughout the study area, ranging between 7 and 14 km. The implication of the structural models is that relatively small amounts of shortening are interpreted to have occurred across these near-surface structures and that the amount of relative uplift is large. This model suggests that basin inversion may be the cause of the structural styles illustrated in this paper.

Data presented supports a tectonic model for the central Kutai Basin involving inversion of a deep Palaeogene rift basin which gives rise to apparent broad regional folding of the shale rich over-pressured section. Closer spaced folding in the near surface, normally pressured and, thus, less ductile deltaic and shelf section of the Samarinda Anticlinorium, is a result of the same inversion.