Tectonostratigraphic Evolution of the Lower Magdalena Basin, Colombia: An Example of an Underfilled to Overfilled Forearc Basin

Abstract

Previous studies along the Andean subduction zones of South America and other areas have shown forearc basins can develop over shallow-dipping subduction zones where subduction dips of approximately horizontal to 15°, alternating with more steeply-dipping (>30°) subduction zones along strike parallel distances of 400 to 1500 km. This study describes the Cenozoic structural and depositional history of the Lower Magdalena basin (LMB), which is the 42,000 km² forearc basin overlying the zone of shallow subduction; the depth to the top of the slab ranges in depth from 30–90 km beneath the LMB. Using 7000 km of seismic reflection lines tied to 33 wells, we describe the initial Oligocene subsidence of the forearc basin along normal faults striking 70–110° that remained active until the Early Miocene. During this period the LMB was filled by 1–3 s TWT (~1500 m) of shallow marine and deep marine facies. During the Middle Miocene, the basin was filled by marine sediments deposited in water depths of 200 to 2600 m. An angular unconformity spanning the interval of 11 to 7 Ma marks an important uplift event of the Sinu belt accretionary prism that emerged as a forearc high at this time. The regional structure of the basin was as a broad syncline that affected older units than the Early Miocene. After the Late Miocene–Pliocene, the forearc high continues to elevate and separate the LMB from the outer Sinu prism. LMB overfills with sediments of shallow marine facies and spills offshore into the proto-delta of the Magdalena fan; these spilled sediments lead to rapid tectonic accretion and growth of the inner and outer Sinu accretionary prism from 5 Ma to recent times. During the period of Oligocene to Early Miocene, different structural styles and magmatic intrusions suggest that the Caribbean plate was subducting at an angle greater than 30°, and that dip decreased to the modern values of 4–8° during the Late Miocene. Comparison of the segmented dip of the ~400 km long subducting Caribbean slab, is consistent with the upper, ~220 km long, shallow-dipping part subducting at rates of 20 mm/yr from 11 Ma (i.e., late Middle Miocene) to recent. We propose that this change from the steeper to shallower-dipping slab in the Middle Miocene led to: (1) increasing elevation of the forearc high of the LMB; (2) the synclinal structure of the LMB; and (3) the possible elevation of the entire LMB as it changed from deepwater marine environments to shallow-water marine and fluvial environments after 11 Ma.

AAPG Datapages/Search and Discovery Article #90219 © 2015 GCAGS, Houston, Texas, September 20-22, 2015