SEDIMENT ACCUMULATION ON TOP OF THE ANDEAN OROGENIC WEDGE - OLIGOCENETO LATE MIOCENE BASINS OF THE EASTERN CORDILLERA, SOUTHERN BOLIVIA

Authors
Citation
Bk. Horton, SEDIMENT ACCUMULATION ON TOP OF THE ANDEAN OROGENIC WEDGE - OLIGOCENETO LATE MIOCENE BASINS OF THE EASTERN CORDILLERA, SOUTHERN BOLIVIA, Geological Society of America bulletin, 110(9), 1998, pp. 1174-1192
Citations number
101
Language
INGLESE
art.tipo
Review
Categorie Soggetti
Geosciences, Interdisciplinary
ISSN journal
0016-7606
Volume
110
Issue
9
Year of publication
1998
Pages
1174 - 1192
Database
ISI
SICI code
0016-7606(1998)110:9<1174:SAOTOT>2.0.ZU;2-F
Abstract
A large volume of Tertiary synorogenic sediment accumulated on top of the Eastern Cordillera of southern Bolivia as the Andean orogenic wedg e was shortened, thickened, and uplifted. Oligocene to upper Miocene s trata were deposited in five basins that were separated by active, nor th-trending, fold-thrust structures of the then-frontal part of the or ogenic wedge. These coarse-grained deposits recorded accumulation in t he most proximal sector of the Andean foreland basin system, the wedge -top depozone. Analyses of depositional systems, sediment dispersal pa tterns, and clast provenance of 0.6-2.3-km-thick, Oligocene to upper M iocene wedge-top successions demonstrate that fault-propagation and fa ult-bend folds commonly isolated individual basins while serving as pr imary 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 stream s were rarely able to cut across growing folds to connect adjacent bas ins. Growth strata and crosscutting and onlapping relationships betwee n contractional structures and wedge-top strata delineate the chronolo gy of fold-thrust deformation in the Eastern Cordillera, Five new Ar-4 0/Ar-39 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 di splacement on two east-vergent thrusts is determined by Ar-40/Ar-39 ag es of tuffs within adjacent growth strata sequences of the Tupiza Form ation conglomerate (16.14 +/- 0.06 Ma) and Oploca Formation (13.33 +/- 0.15 Ma, 15.7 +/- 2.4 Ma). These ages, combined with basin deposition al histories, demonstrate synchronous and out-of-sequence thrust displ acement during Miocene shortening. Upper-crustal contractional deforma tion and wedge-top deposition terminated in the Eastern Cordillera dur ing 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, origi nally developed shove the toe of the orogenic wedge at relatively low elevations, are now >250 km west of the active thrust front and at sim ilar to 3 km elevation, The modern wedge-top depozone overlies the act ive frontal part of the orogenic wedge and consists of strata in thrus t-bounded, Subandean Zone basins and sediment overlying blind structur es beneath the westernmost Chaco Plain. In general, wedge-top deposits become highly susceptible to erosional recycling as the orogenic wedg e propagates forward and the wedge surface is uplifted. Nevertheless, Eastern Cordillera wedge-top deposits have been preserved for similar to 10-30 m.y. in southern Bolivia, Such long-term preservation may ref lect the inability of drainage systems to remove sediment mass from th is low-precipitation region of the central Andes, Retention of sedimen t mass within the orogenic belt may promote critical thrust-wedge cond itions in which propagation of the thrust front is favored.