2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 254-1
Presentation Time: 1:45 PM


CAMPBELL, Kenneth, Vertebrate Zoology Department, Natural History Museum of Los Angeles County, 900 Exposition Blvd, Los Angeles, CA 90007, kcampbell@nhm.org

In the Amazon Basin and adjacent oceans, the late Miocene of ~10.5-9.5 mya saw the formation of many geological features that pose giant mysteries today. Some of these features are: (1) the basin-wide Ucayali Unconformity, an erosional unconformity that forms the sharp contact between the older Miocene formations and the upper Miocene-Pliocene capping formation (Madre de Dios Fm. in Peru; Içá Fm. in Brazil); (2) giant clasts with dimensions commonly greater than 1-2 m that occur in localized conglomeratic deposits on opposite sides of the basin, immediately above the Ucayali Unconformity; (3) an abrupt end, marked by the Ucayali Unconformity, to the long-lived and widespread Pebasian mega-lake system of central Amazonia; (4) an undulating contact in the form of giant ripple marks between lower and upper horizons of presumed upper Miocene deposits in widely separated regions, from central eastern Amazonia to Roraima, northern Brazil; (5) large-scale fluvial bars covered with large clasts and region-wide mega-ripple marks in the savannas of Roraima, Brazil; (6) an erosional surface of regional proportions that occurs on the Amazon Fan, and a possible comparably-aged, regional erosional surface that occurs on the Demerara Plateau in the Atlantic off Suriname; and (7) an exceptionally large, temporary influx of terrestrial and carbonate sediments that settled on the Ceara Rise in the Atlantic Ocean. Further, (8) many species of giant vertebrates disappeared at this time, their last occurrence recorded in the conglomeratic basal horizons of the Madre de Dios/Içá Formation; and (9) it is at ~10.5-9.5 mya that the Andes began their spectacular rise from near sea level to reach near modern levels by 6 mya, a seemingly vertical rise that appears more likely to have resulted from a change in buoyancy of the Andean core than compression resulting from tectonic plate movements. Individually, the formation of each of these features requires an exceptional geological event. But is the occurrence of all of these features in the same time frame really just a coincidence, or are they linked? If linked, it must be assumed that an as yet unidentified geologic event on an extraordinary scale must have occurred.