Profil, Vol. 2

Tiefwasser-Sedimentationsprozesse und -produkte in den Forearc-Becken des mittelamerikanischen Inselbogensystems: eine sequenzstratigraphische Analyse

Jutta Winsemann

This work deals with the sequence stratigraphic interpretation of Upper Cretaceous and Lower Tertiary turbidite systems in southern Central America. These deep-water sediments were deposited in the forearc area of the Central American island arc, and today comprise the main body of information regarding the former island arc system. It should be possible to reconstruct the early history of the island arc from a structural point of view based on the architecture and the composition of components of the turbidite systems. It should also be possible to analyse the dynamics of the sedimentation from the forearc basins, which could then serve as a model for island arc systems in general.

The complex, pre-Campanian history of northern Costa Rica results from the incorporation of allochthonous forearc rocks, 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 early Cretaceous times before the southerly Chortis block. The deactivation of this subduction system took 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 system 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 onto the primitive island arc rocks of northern Costa Rica. Owing to this structural evolution of the Central America area the origin of the thickened Caribbean crust ("Horizon B") cannot be explained by the passive insertion of a pacific oceanic plateau. The Caribbean sill event is therefore interpreted to be a parautochthonous formation 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 forearc basins, which can be correlated with the cycles ZC-4, TA1, TA2. TA3, and TA4 (cf. HAQ et al., 1988). Although thick submarine fan systems have been documented in the forearc basins of south-western Nicaragua and northern Costa Rica as far as the Eocene, the south-lying island arc segments (southern Costa Rica, Panama) are mainly characterized by hemipelagic sedimentation systems which, as a result of basaltic lava, breccias and chaotic mass flow deposits, are indicative of a fault-controlled environment 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 northerly and southerly island arc segments of Costa Rica and Panama only demonstrate a similar sedimentatary development from the middle Eocene, thereby signifying the formation of a continuous inter-American subduction zone. The internal architecture of the depositional sequences is mainly characterized by the formation of repetitive lowstand systems tracts. Either sandrich tabular lobe systems (Type 1, cf. MUTTI & NORMARK, 1987) or channel-lobe systems of smaller dimensions (Type II, cf. MUTTI & NORMARK, 1987) developed, depending on the supply of sediment and the tectonics. These in turn were overlain by thick channel-overbank complexes or prograding wedges (Type III, cf. MUTTI & NORMARK, 1987). The plant material and resedimented neritic fossil content is generally high in lowstand systems tract deposits. The composition of these varies in accordance with the position of the relative sea level. The depositional systems of the transgressive and highstand systems tracts could only be segregated in areas of low sedimentation rates. In these areas, it 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.