科研之友
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摘要
Detrital zircon U-Pb geochronology has been used extensively to develop provenance histories for surface outcrops of key stratigraphic localities within sedimentary basins. However, many basins lack sufficiently continuous and widespread exposures of complete successions to evaluate proposed long-term tectonic histories, stratigraphic correlations, and paleodrainage patterns within individual basins. Here, we demonstrate the utility of subsurface detrital zircon U-Pb analysis by integrating ages from three key wells (21 subsurface samples) with previously reported data from six exposed intervals (90 surface samples) within a single basin. Samples from the 5-10-km-thick clastic successions span several structural blocks over an similar to 300 x 50 km swath of the Middle Magdalena Valley Basin, a north-trending intermontane basin in the northern Andes of Colombia. Available U-Pb age distributions for modern rivers highlight the distinctive signatures of several competing sediment sources, including two major contiguous ranges (Central Cordillera and Eastern Cordillera) and two localized block uplifts (Santander Massif and San Lucas range). U-Pb results from Jurassic through Neogene stratigraphic units spanning the nine surface and subsurface sites, including several type localities, enable comparisons of provenance shifts at specific sites and spatial variations among key stratigraphic intervals across multiple sites. Distinctive age populations for the Andean magmatic arc, retroarc fold-thrust belt, and South American craton facilitate correlation of stratigraphic units and reconstruction of the long-term provenance and tectonic evolution of the Middle Magdalena Valley Basin. Nearly all surface and subsurface localities show up-section changes in age spectra consistent with (1) Jurassic growth of extensional subbasins fed by local igneous sources, (2) Cretaceous deposition in an extensive postrift setting, and (3) protracted Cenozoic growth of basin-bounding ranges during Andean crustal shortening. Subsurface samples augment surface samples, highlighting their utility in developing regional source-to-sink relationships, the timing of paleodrainage integration, and tectonic reconstructions. Provenance shifts of mid-Paleocene and latest Eocene-earliest Oligocene age are consistent with incipient uplift of the flanking Central Cordillera and Eastern Cordillera, respectively. However, a well-documented phase of latest Paleocene-middle Eocene beveling of basement uplifts in the Middle Magdalena Valley Basin appears to be largely aliased in the detrital record. Moreover, despite the proximity of the magmatic arc, there is insufficient syndepositional evidence for a proposed Paleogene pulse of magmatism and, in this case, limited utility of U-Pb ages in pinpointing precise depositional (stratigraphic) ages. U-Pb age spectra for Oligocene through Pliocene basin fill underscore complex along-strike (north-south) and cross-strike (east-west) variations reflective of compartmentalized transverse deposystems demarcated by point-source contributions from the Central Cordillera and Eastern Cordillera. The late Miocene appearance of 100-0 Ma grains and a regional switch to broad, multimodal age distributions suggest the initial integration of the longitudinal proto-Magdalena River, linking the Middle Magdalena Valley Basin with southern headwaters in the Upper Magdalena Valley and likely driving increased sedimentation rates farther north in the offshore Magdalena submarine fan of the southern Caribbean margin.
- 出版日期2015-12
- 单位兰州大学
