Sediment accumulation on top of the Andean orogenic wedge: Oligocene to late Miocene basins of the Eastern Cordillera, southern Bolivia

Abstract

Research Article| September 01, 1998 Sediment accumulation on top of the Andean orogenic wedge: Oligocene to late Miocene basins of the Eastern Cordillera, southern Bolivia Brian K. Horton Brian K. Horton 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1998) 110 (9): 1174–1192. https://doi.org/10.1130/0016-7606(1998)110<1174:SAOTOT>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Brian K. Horton; Sediment accumulation on top of the Andean orogenic wedge: Oligocene to late Miocene basins of the Eastern Cordillera, southern Bolivia. GSA Bulletin 1998;; 110 (9): 1174–1192. doi: https://doi.org/10.1130/0016-7606(1998)110<1174:SAOTOT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A large volume of Tertiary synorogenic sediment accumulated on top of the Eastern Cordillera of southern Bolivia as the Andean orogenic wedge was shortened, thickened, and uplifted. Oligocene to upper Miocene strata were deposited in five basins that were separated by active, north-trending, fold-thrust structures of the then-frontal part of the orogenic wedge. These coarse-grained deposits recorded accumulation in the most proximal sector of the Andean foreland basin system, the wedge-top depozone.Analyses of depositional systems, sediment dispersal patterns, and clast provenance of 0.6–2.3-km-thick, Oligocene to upper Miocene wedge-top successions demonstrate that fault-propagation and fault-bend folds commonly isolated individual basins while serving as primary sediment sources. Growth strata which formed by progressive fold-limb rotation indicate thrust-fault displacement and related folding concurrent with deposition. Alluvial fans defined most basin margins, whereas braided streams or small lakes occupied basin axes. Diagnostic stratigraphic units confined to individual basins suggest that streams were rarely able to cut across growing folds to connect adjacent basins.Growth strata and crosscutting and onlapping relationships between contractional structures and wedge-top strata delineate the chronology of fold-thrust deformation in the Eastern Cordillera. Five new 40Ar/39Ar dates and previously published K-Ar dates, ranging from 30 to 8 Ma, define an Oligocene phase of west-vergent backthrusting followed by primarily east-vergent thrusting during Miocene time. Timing of displacement on two east-vergent thrusts is determined by 40Ar/39Ar ages of tuffs within adjacent growth strata sequences of the Tupiza Formation conglomerate (16.14 ± 0.06 Ma) and Oploca Formation (13.33 ± 0.15 Ma, 15.7 ± 2.4 Ma). These ages, combined with basin depositional histories, demonstrate synchronous and out-of-sequence thrust displacement during Miocene shortening.Upper-crustal contractional deformation and wedge-top deposition terminated in the Eastern Cordillera during late Miocene time as the thrust front propagated eastward into the Subandean Zone. Continued thrust-front migration produced the present-day configuration in which Eastern Cordillera wedge-top basins, originally developed above the toe of the orogenic wedge at relatively low elevations, are now >250 km west of the active thrust front and at ∼3 km elevation. The modern wedge-top depozone overlies the active frontal part of the orogenic wedge and consists of strata in thrust-bounded, Subandean Zone basins and sediment overlying blind structures beneath the westernmost Chaco Plain.In general, wedge-top deposits become highly susceptible to erosional recycling as the orogenic wedge propagates forward and the wedge surface is uplifted. Nevertheless, Eastern Cordillera wedge-top deposits have been preserved for ∼10–30 m.y. in southern Bolivia. Such long-term preservation may reflect the inability of drainage systems to remove sediment mass from this low-precipitation region of the central Andes. Retention of sediment mass within the orogenic belt may promote critical thrust-wedge conditions in which propagation of the thrust front is favored. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.