Upper Jurassic calcareous nannofossils are present at ODP Leg 173 Sites 1065 and 1069, where they overlie presumed continental basement blocks now buried under the IAP. The youngest Jurassic assemblages at each site are Tithonian in age, the same as those at nearby Leg 149 Site 901. Those at Site 1065 are the oldest (early to mid-Tithonian in age), whereas those at Sites 901 and 1069 are latest Tithonian in age, having been deposited close to the Jurassic/Cretaceous boundary.
The paucity of the assemblages, the prevalence of coccospheres, and the relatively high organic contents of the fine-clastic sediments in which they occur are characteristic of a restricted interior basin, which in this case had little communication with the open ocean until the latest Tithonian, when the first nannofossil oozes appear as thin laminae (less than ~2 mm thick) at Sites 901 and 1165. These provide the first hints of better communication with the open sea; the bottom sediments, however, remained dysaerobic.
The restricted interior basin was shattered during the major rifting episode (a Berriasian event), during which sustained open marine conditions were established, resulting in the deposition of nannofossil chalks. As a consequence of this rifting, the Jurassic sequences were dispersed along with their underlying basement blocks of presumed continental crust across the ocean-continent transition of the IAP, probably as a result of detachment faulting.