EXTENSIVE LATE HOLOCENE PEAT DEPOSITS IN THE ORINOCO DELTA, VENEZUELA–A MODERN ANALOG FOR COAL DEVELOPMENT IN A TROPICAL DELTA

The Orinoco Delta comprises 22,000 km² of fresh- to brackish-water wetlands within the seasonally wet and dry tropical northeastern South America. Preliminary mapping in the delta reveals approximately one third of the delta plain is underlain by peat, ranging in thickness from 3 to 10 m. Mapping of vegetation communities in the northwestern delta indicates that peat develops in both forested and herbaceous wetland settings. Peat accumulation appears to be greatest in marine-influenced portions of the delta plain and towards the centers of the broad interdistributary-channel basins.

About 80 percent of Orinoco River water and sediment is discharged by the Río Grande and Caño Araguao distributary channel systems in the southern delta. Distributaries (caños) in the central and northwestern delta are black-water systems that transport little terrigenous sediment, so that this portion of the delta plain is sediment starved. Hydrodynamics in the central and northwestern delta-plain are controlled, to varying degrees, by the pronounced seasonal flood pulse of the Orinoco River, direct rainfall, and tides (which have amplitudes of about 0.9 m in the central delta). Holocene subsidence rates are estimated to be 0.8 to 2.0 mm/yr for the middle delta, and from 2.2 to >6.0 mm/yr for the lower delta plain.

Peat accumulation in the Orinoco Delta was not as widespread in the early and middle Holocene, although vegetal remains in sediments were commonly a major component. Reconstruction of the Holocene paleogeography indicates that rapid progradation of the delta coast, promoted by large and continuous influx of Amazon sediment, coupled with restriction of sediment discharge to the southern delta, lead to sediment-starved conditions in the central and northwestern delta plain. Tides, ample rainfall, and the pronounced Orinoco River flood pulse, in conjunction with subsidence and steady to slowly rising sea levels, maintained perennially saturated conditions in that part of the delta.

Holocene to modern peat is also widespread along the Guyana and Suriname (Guiana) coastal plain adjacent to and southeast of the Orinoco Delta. Studies of the Orinoco Delta and Guiana coastal plain would provide valuable insight into environmental conditions conducive to widespread peat, and ultimately coal, development.