Early History of the Orinoco Delta on the Northeastern Margin of South America

Castillo, Karilys¹ and Mann, Paul
¹[email protected]

The 12-km-thick Orinoco delta in the Atlantic Ocean of northeastern South America is formed by the Orinoco River which is the seventh largest river in the world and drains an 880,000 km² area that extends across the continent to the northwestern Andes. This study uses 620 km² of 3D seismic, 650 km of 2D seismic, and 14 wells with well logs from the Punta Pescador area of northeastern Venezuela near the Columbus Channel and border with Trinidad combined with maps from previous subsurface studies to the west (8500 km²) and east (10000 km²) in Venezuela and to the southeast in Guyana (3000 km²) to reconstruct the margin geometry and paleogeography prior to the arrival of the Orinoco delta in its present location. The main seismic sequences in the northern area in Venezuela consist of: 1) transgressional, 1.2 to 3-km-thick Lower Miocene marine sandstone and shale deposited in a foreland basin setting and with a basal foredeep unconformity dated approximately 25.5 Ma and a top defined by the Middle Miocene unconformity dated at 16.5 Ma; 2) regressional late Miocene shallower water marine sandstone and shale overlying a major erosional unconformity marked by deepwater channels on the slope filled by shale; previous workers have correlated this unconformity with the latest Miocene (Messinian) eustaticlow stand event. In the southern area, the margin of Guyana exhibits a similar late Miocene erosional event that we correlated with this latest Miocene event. Based on correlations of these major units and unconformities along the margin, we propose the following sequence of events affecting the development of the Orinoco delta: 1) ponding of fluvial-deltaic sediments of the Orinoco fluvial system in the restricted marine embayment of the Eastern Venezuelan foreland basin during the Oligocene and Miocene; the onset of major input of the Orinoco in this area is throught by most workers to be Late Miocene in age; 2) the late Miocene event lowered sea level and produced a major erosional event that breached a continental high that allowed the Orinoco delta to suddenly prograde into the deeper water Atlantic area in the earliest Pliocene; and 3) early Pliocene to recent progradation of the Orinoco delta is well documented into the deeper water areas of Trinidad and offshore Venezuela.

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013