A LATE-MIOCENE DELTA-LACUSTRINE SYSTEM IN THE EASTERN SOLIMÕES BASIN: PRELUDE TO THE MODERN AMAZON RIVER

Post-Cretaceous events related to the Andean Orogeny led to formation of the modern Amazon drainage. Although Andean related events are recorded in Miocene deposits in the western Amazon Basin, few studies have focused on the eastern portion of the Solimões basin where deposits onlap onto the Purus Arch, the boundary with the Amazonas Basin to the east. Outcrop-based facies analysis along the Solimões River reveals eleven sedimentary facies which were grouped in four associations: 1) lacustrine prodelta: consists of pelites and subordinate sandstone drapes with frequent slumps and load cast structures that thicken eastward toward the Purus Arch. 2) Lacustrine delta front: comprised of proximal and distal mouth bar deposits. The first consists of complex cross-stratified sandstone that include several structures (i.e. planar undulated to sigmoidal cross stratification and climbing ripple cross lamination cut by growth faults and slumps) and frequently shows east inclined foresets. Distal mouth bar/crevasse deposits consist of sandstones interbedded with pelites rich in vegetal debris and void of bioturbation. 3) Delta plain: consist of medium-scale heterolithic inclined stratified sandstone (interpreted as distributary channel deposits), pelite with vegetal debris (related to infill of abandoned channels), and bioturbed coarsening upward successions of sandstone and pelite (interpreted as crevasse splay deposits). 4) Meandering fluvial channel: represented by point bar deposits consisting of large-scale inclined heterolithic stratified sandstone and pelite with vegetal debris interpreted as abandoned channels. This lacustrine deltaic system with fluvial meandering feeder was restricted to the Solimões basin and prograded against to the western flank of the Purus Arch during the late-Miocene. This interpretation provides a new paleogeographic model that identifies the Purus Arch as a geographic barrier from late- Miocene to Pleistocene time that controlled sedimentation and/or connection of the Solimões-Amazon drainage. The absence of definitively Andean detrital zircon in mid-to-late Miocene fluvial deposits of the Amazon Basin (Mapes et al., this meeting) coeval to the Purus Arch divide suggest that Solimões-Amazonas connection occurred as late as the youngest Neogene.