哥倫比亞西北部下馬格達萊納盆地構造及地層架構

Abstract

Lower Magdalena Valley位於南美洲哥倫比亞西北角，由北部的Plato次盆地及南部的San Jorge次盆地共同組成，堆積漸新世-更新世沉積物。本研究利用三條與盆地走向大致垂直之西北-東南向震測剖面，及綜合一百餘口井下地質資料完成之各地層沉積環境重建圖，進行盆地構造及地層架構分析，研究成果發現Lower Magdalena Valley西側的San Jacinto逆衝斷層摺皺帶上方發育的豬背盆地，指明在逆衝斷層帶間形成凹陷，接受沉積物充填，可視為前陸盆地系統的楔形體上方堆積區，東側Lower Magdalena Valley內部顯示早期陸緣分裂形成之正斷層反轉，逆衝斷層上盤形成褶皺構造，後期沉積物大致呈水平堆積，向西側San Jacinto摺皺帶抬升減薄; Lower Magdalena Valley沉積演化與典型前陸盆地相似，盆地於早中新世沉積環境突然加深，形成半深海環境，對應前陸盆地演育的未充填(Underfilled)階段; 晚中新世之後，盆地沉積環境逐漸變淺，轉變以河流相為主之陸地沉積環境，對應前陸盆地演育的過充填(Overfilled)階段。研究結果顯示Lower Magdalena Valley之構造架構及沉積地層序列，基本上符合前陸盆地特徵，故推論Lower Magdalena Valley為一前陸盆地。

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.