Profil, Vol. 4

Hubert Amann

Randmarine und terrestrische Ablagerungsräume des neogenen Inselbogensystems in Costa Rica (Mittelamerika)

1 - 161

Institut für Geologie und Paläontologie, Universität Stuttgart, 1993

Abstract

The results of this sedimentological facies analysis are a contribution to the geological in-vestigation of the southern Centralamerican island arc system. The main objective of this study is to compare the Neogene sedimentation of fore-arc areas with those of back arc areas using the Baja Talamanca Trough and Punta Carballo Basin as examples.

During the time-span in question the depositional geometry of the island arc sedimentation was strongly influenced by three major tectonic events: the depositional environment changed from deep to shallow marine on both sides of the island arc after the structural transformation of the Central American island arc during the Late Oligocene. During the Early Neogene a phase of isostatic back arc tensional release occurred. In the Late Miocene this phase culminated in an uprise of plutonic and effusive magmas and the formation of trench faults. When eastern Panama collided with the South American plate at the turn to the Pliocene this development was terminated. Continuous com-pression resulted in the uplift of the Cordillera de Talamanca whose mountain basement was sheared along a thrust belt during the Early Pliocene. By Late Pliocene time the incipient collision of the Cocos Ridge led to steepening of these faults and to nappe transport.

Back arc: Throughout this tectonic phase 3500m thick neritic and litoral sediments representing three depositional sequences of the second order were deposited in the back arc basin. These sequences correlate with the depositional sequences TB 1 (Late Oligocene - Early Miocene), TB 2 (middle Early Miocene and Middle Miocene) and TB 3 (Late Miocene - recent). The lowstand systems tract of the depositional sequence TB 1 consists of the erosional debris of a carbonate ramp which was covered by pelagic and hemipelagic oozes during the deposition of the transgressive and highstand systems tracts. The sea level dropped to the shallow neritic level during the deposition of the lowstand systems tract of the TB 2 sequence, and thus initiated the progradation of deltas. Under conditions of constant subsidence and high sedimentation rates the delta facies persisted until after the deposition of the trans-gressive and high stand systems tracts, and until the deposition of the lowstand systems tract of the sequence TB 3. During the deposition of the transgressive systems tract a change to neritic sedimentation took place during which three retro-grade delta sequences were deposited. Each delta sequence was influenced by eustatic fluctuations of the third order. The following aggradiation marks the high stand systems tract. At the turn of the Pliocene an abrupt sedimentational change occurred which initiated the progradiation of fan deltas in the southern Baja Talamanca Basin. In the northern Baja Talamanca Basin this level is characterized by a pronounced Type 2 sequence boundary. Inter-fingering litoral and paralic delta deposits follow. At the turn to the Late Pliocene an abrupt rise in sea level produced retrograding facies patterns. This scenario corresponds with decreasing relief in the source area.

Fore-arc: In the fore-arc area the depositional sequences are generally less thick than in the back arc. The Punta Carballo Basin exposes the depositional sequences TB 2 and TB 3 with a total thickness of 300m. The depositional sequence TB 2 consists of the Middle Miocene barrier coast sediment facies which characterizes the trans-gressive systems tract. In the following highstand systems tract a delta prograded. The outcrops of the depositional sequence TB 3 similary expose sediments of the transgressive systems tract. In accordance with the back arc three retrograding delta sequences with lapilli components as the coarse fraction were deposited. After an ag-gradiational phase during the deposition of the early highstand systems tract a Type 1 sequence boun-dary marks the turn to the Pliocene. The coarse clastic delta facies follows the tectonic framework of down-thrust fault-bound subsidence which cor-relates with the beginning of the thrust belt for-mation. Lacustrine sediments were deposited in the Late Pliocene along the marginal area of the strongly segmented basin. These sediments were later sheared sinistrally. Their deformational pattern can be explained by the rotation of the south Costa Rica island arc segment which also influenced the thrust geometry of the back arc.

The sequence stratigraphy of both basins de-monstrates synchronous filling which is also sub-stantiated by the correlation of eustatic signals up to the third order depositional cycles. Until the Late Miocene sedimentation was predominantly con-trolled by the above mentioned factors. In the Pliocene the influence of tectonic processes in-creased which controlled basin subsidence and clastic sedimentation in the fore-arc basin. From the Late Pliocene onwards tectonic influence dominated the depositional regime.