Abstract
The cornplex, pre-Campanian history of northem Costa Rica results from allochthonous slabs, which are the remains of an old accretionary prism, which can be traced back to the subduction of proto-Caribbean crust during late Jurassic to Cretaceous times before the southerly Chortis block. The deactivation of this subduction system place in Cenomanian times. It was only when the subduction process taking place there was completed that an inter-American subduction zone was formed, from which the Central American island arc systems later emerged. A left lateral strike-slip-system later developed in the area of the former northern Caribbean subduction zone, represented today by the Hess Escarpment. Thrusts in a southerly direction took place along this transform margin during the Campanian. The remains of the former subduction complex were thus transported southwards, and thrusted to the primitive island arc rocks of northern Costa Rica. Owing to this structural evolution of the Central American are the origin of the thickened Caribbean crust ("Horizon B”) cannot be explained by the passive insertion of an pacific oceanic plateau. The Caribbean sill event is therefore interpreted to be a parautochthonous fomation in the western Caribbean and eastern part of the Phoenix plate.
The basin systems of Central America demonstrate a similar, if not identical development, starting at the late Campanian. Five 2nd order depositional sequences can be identified within the foreaar basins, which can be correlated with the cycles ZC-4, TA l, TA 2, TA 3, and TA 4 (HAQ et al., 1988). Although thick submarine systems have been documented in the forearc basins of south-western Nicaragua and northem Costa Rica as far as the Eocene, the south-lying island are segments (southern Costa Rica, Panama) are mainly charcterized by hemipelagic sedimentation systems which, as a result of basaltic lava, breccias and chaotic mass flow deposits, are indicative of a fault-controlled enviroment in the area of a transform fault. This transform fault may have been the link to island arc segments lying further south which collided with the Central Cordillera of South America during late Cretaceous to early Tertiary times. The northely and southerly island are segments of Costa Rica and Panama only demonstrate a similar sedimentary development from the middle Eoceno, thereby signifying the formation of a continuous inter-American subduction zone.
The internal architecture of the depositional sequences is mainly charcterized by the fomation of repetitive lowstand systems tracts. Either sandrich tabular lobe systems (Type I, cf. MUTTI & NORMARK, 1987) or channellobe systems of smaller dimensions (Type II, cf., MUTTI & NORMARK, 1987) developed, depending on the supply of sediment and tectonics. These in tum were overlain by thick channel-overbank complexes or prograding wedges (Type III, cf. MUTTI & NORMARK, 1987). The plant material and resediment neritic fossil content is generally high in lowstand systems tract deposits. The composition of these varies in accordance with the position of the relative se level. The depositional systems of the transgressive and highstand systems tracts can only be segregated in areas of low sedimentation rates. In these areas, il typically consists of fine-grained calcareous deposits. The fast rise and highstand of the sea level is therefore documented through the progressive increase in the carbonate content.