| Summary: | 碩士 === 國立臺灣大學 === 海洋研究所 === 103 === Lower Magdalena Valley is located in the northwest corner of Colombia of South America. It consists of the northern Plato sub-basin and the southern San Jorge sub-basin, filled with Oligocene to Pleistocene sediments. Using three seismic reflection profiles oriented in the northwest-southeast direction and approximately perpendicular to the regional strike, and depositional environment reconstruction maps based on the well logs and core data, this paper intends to establish the structural and stratigraphic frameworks of the Lower Magdalena Valley.
The results of study show that: (1)The presence of piggyback basins in the San Jacinto fold-and-thrust belt located to the west of the Lower Magdalena Valley, indicates that sediments within the piggyback basins are deposited in the wedge-top depositional zone. (2)Inversion structures including basement-involved normal faults inherited from the pre-orogenic passive margins are reactivated as thrust faults. (3)Fault-propagation folds formed at the tip of a thrust fault. (4)Younger sediments are accumulated to form horizontal layers and thin toward San Jacinto fold-and-thrust belt since upper Miocene. (5)The progressive change of depositional environment of Lower Magdalena Valley are similar to that of foreland basin. During lower Miocene, the basin was characterized by bathyal environment belonging to an underfilled stage. Since upper Miocene, the basin has been filled with sediments, resulting upward coarsening sequence from bathyal to fluvial, the basin was characterized by fluvial environment reaching an overfilled stage.
The structural and stratigraphic frameworks of Lower Magdalena Valley are similar to that of foreland basins. Therefore, the study infers that Lower Magdalena Valley can be considered as a foreland basin.
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