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Distribution of rocks at and below the surface
Published in A.C. McLean, C. D. Gribble, Geology for Civil Engineers, 2017
If subsidence has increased the area of deposition of the upper formation, then in the fringe of the area, where bed K was not deposited, bed L rests directly on the older formation E-A, and is said to onlap or overlap K (Fig. 4.28). Onlap is always accompanied by overstep. The variable gap (with respect to the fullest sequence of strata for the area) at the unconformity is a combination of the absence of the top beds of the lower formation because of overstep and the absence of the bottom beds of the upper formation because of onlap.
Inversion history of the northern Tasman Ridge, Taranaki Basin, New Zealand: implications for petroleum migration and accumulation
Published in New Zealand Journal of Geology and Geophysics, 2020
Hannu Seebeck, Glenn Paul Thrasher, Graham Paul Viskovic
The deformation of strata above a Late Miocene foreset horizon (N44) and the age of strata preserved beneath the base Pliocene unconformity across inversion structures (10–8.5 Ma) indicate the development and deformation of the base Pliocene unconformity is consistent with positive inversion and/or increased contraction rates across the southern Taranaki Basin between 8 and 6 Ma. (e.g. Kamp and Green 1990; King and Thrasher 1996; Crowhurst et al. 2002; Vonk and Kamp 2008; Reilly et al. 2015). South-directed onlap and marine transgression immediately followed inversion and uplift (Vonk and Kamp 2008). To the north of the study area, the earliest strata overlying this unconformity are c. 7.2 Ma (late Tongaporutuan) in age (Vonk and Kamp 2008), becoming progressively younger towards the south where Early Pliocene strata immediately overlie the base Pliocene unconformity (Shell Todd Oil Services 2012).