The Obispo Turbidite Ramp—a Sedimentary System Immune to Sea-Level Changes
The Ordovician Obispo Formation in southern Bolivia is a mud-dominated turbidite succession with less than 30% sand content. It was deposited on a westward-inclined shelf downdip from a delta draining the Brazilian Shield. The succession is characterized by sheet-sandstone bodies built up of stacked decimeter-thick turbidites intercalated into prominent intervals of laminated mudstones. The Obispo Formation lacks distinct grain size variations: throughout the unit there is no stratigraphic interval where sandstone packages agglomerate or mudstones occur exclusively. Only centimetre-thick layers of bioturbated siltstones are restricted to proximal parts of the system. The sheet-sandstone bodies are interpreted as lobes based on geometries and facies. No feeder channels developed. The sand was transported to this ramp by unconfined turbidite flows. Sedimentary stacking patterns suggest that a line rather than a point source has supplied the turbidites. The absence of grain-size trends is attributed to the mud-rich character of the sediment. It enabled turbidite flows to distribute their sand load in proximal as well as distal portions of the basin throughout its evolution. The lack of well-developed proximal-distal criteria also complicates the reconstruction of relative sea-level fluctuations from this succession. The intercalation of prodelta bioturbated facies in proximal ramp areas is the only lithological variation that reflects relative sea-level falls. This study shows that mud-rich turbidite ramps can be built of only lobes and interlobe mudstones without channel-levee systems. The irregular stacking patterns and lack of distinct grain-size trends argues for shedding of sand onto the ramp independent of relative sea-level positions.
AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005