7) Plio-Pleistocene Palynostratigraphy of the Amazon Fan, Offshore Brazil

KEITH RICHARDS (1) and STEPHEN LOWE  (2)
(1) KrA Stratigraphic, Deganwy, UK
(2) BP Exploration,  Sunbury on Thames, UK

EXTENDED POSTER ABSTRACT: Presented at The Micropalaeontological Society (TMS) and American Association of Stratigraphic Palynologists (AASP) joint meeting, London, England, September 2002

PLIO-PLEISTOCENE PALYNOSTRATIGRAPHY OF THE AMAZON FAN, OFFSHORE BRAZIL:
INSIGHTS INTO VEGETATION HISTORY, PALAEO-CLIMATE & SEQUENCE STRATIGRAPHY

THE AMAZON FAN
The Amazon Fan is approximately 10 million years old and was formed after Andean uplift gave rise to the Amazon river system. This study presents a summary of biostratigraphic results from Pleistocene and Late Pliocene sediments from several wells drilled in deep water on the fan. Foraminifera indicate that the studied sections were deposited in deep shelfal or bathyal settings. Age dating is based mainly on nannofossils with peak abundances, in most cases, equating with maximum flooding surfaces or lowstand condensed sections. Sequence boundaries are suggested at 1.4 Ma (Early Pleistocene), 1.9 Ma, 2.4 Ma, 2.6 Ma and 3.0 Ma (Late Pliocene).

Calcareous nannofossils show considerable variations in abundance, with peaks corresponding to maximum flooding surfaces which are correlatable between wells. Foraminiferal assemblages include varied abundances of planktonics and deep-water benthonics. These give independent age control and precise palaeo-water depth.
 The palynological records include highly diverse pollen and spore assemblages reflecting the wide range of vegetation types in the Amazon catchment. Marine dinocysts are locally common but may be transported from the marine shelf into deeper water. Mangrove pollen occur only in low to moderate numbers, probably due to continually strong freshwater influence. Abundance peaks of freshwater algae indicate times of maximum freshwater discharge. Reworked elements are present in most samples but are of limited stratigraphic value.
Fern spores, grass pollen and cold tolerant pollen types such as Podocarpus and Alnipollenites verus are frequent in some intervals and indicate that a large proportion of the palynoflora originates from Andean and sub-Andean localities. This type of river discharge is referred to as "whitewater", reflecting a high sediment load, mostly of fine clastics.

CHARRED GRAMINEAE (GRASS) CUTICLE
Several intervals within the Early Pleistocene and Late Pliocene contain increased quantities of charred Gramineae (grass) cuticle which probably originates mainly from "cerrado"(wooded grassland) or semi-deciduous woodland. These are mostly dry, fire-prone localities found mainly to the south and east of the Amazon catchment at the present time, where Paleozoic rocks of the Brazilian Shield are often exposed. Peak abundances of charred cuticle in the sedimentary record are thought to result from periods of very dry climate, after which seasonal floods rejuvenated dry or ephemeral rivers: these are "clearwater" rivers which carry mostly coarser clastics, including quartz-rich sands eroded from the Brazilian Shield.

Charred Gramineae (grass) cuticle is produced during times of fire in the savannah and "cerrado". Abundance trends have proven value in well correlation in the Amazon Fan, and in other areas such as the Niger Delta. The cuticle is found most commonly in the Amazon Fan in sediments of Late Pliocene age, suggesting that this was a period of mostly dry climate and frequent fires. The highest abundances of cuticle may also coincide with sea level lowstands.

A POSSIBLE DEPOSITIONAL, CLIMATIC AND SEQUENCE STRATIGRAPHIC MODEL?
The palynological records from the Amazon Fan provide an excellent basis for well correlation and also give an insight into the nature of vegetation changes that have taken place within the Amazon catchment. In many cases, there is evidence for "cyclicity" in vegetation change that is mirrored in the present-day natural vegetation. The major control is likely to be climate which may, in turn, be linked to sea level.

RIVER TYPES IN THE AMAZON CATCHMENT

"CLEARWATER RIVERS"    e.g. Rio Tapajos & Rio Xingu
Sourced mainly in higher ground of the Brazilian Shield to the north and south of the main Amazon River. These rivers drain areas with a more seasonal climate and are likely to have more variable, seasonal flow. Their "clearwater" character is an indication of low amounts of clay in suspension. Predicted to carry significant amounts of coarser clastics into the Amazon Fan during times of extreme seasonal flow.

"BLACKWATER RIVERS"
e.g. Rio Negro
Probably of limited geographical extent. Sourced mainly in very wet forest areas and have "blackwater" as a result of leached humic acids. Influence upon the Amazon Fan probably minimal.

"WHITEWATER RIVERS"
e.g. Rio Solimoes (Amazon)
Sourced mainly from the Andean foothills and carry a high sediment load of fine clays and silts, giving a "whitewater" appearance. These rivers probably carry mixed palynological components derived from the entire Amazon catchment, but include sub-montane and temperate pollen and spores such as Alnipollenites verus and Podocarpus. Sediments carried into the Amazon Fan are predicted to be mostly non-reservoir.

FIRE IN THE AMAZON BASIN: AN "EL NINO" PHENOMENON?
The effects of the "El Nino-Southern Oscillation" (ENSO) phenomenon in South America are well documented. In ENSO years, the Amazon catchment usually becomes drier (and more prone to fire) as a result of shifting climatic cells. The palynological evidence from the Amazon Fan confirms that fires were a feature of the climatic and vegetation cycle in the pre-Holocene and pre-Quaternary, with the Late Pliocene (approximately 1.8 Ma TO 3.45 Ma) being a time especially prone to fire. The palynological record may hold evidence for the existence of ENSO phenomena on a geological as well as human timescale.
 

CONCLUSIONS
New biostratigraphic data from the Amazon Fan of  Pleistocene and Late Pliocene age indicate that deposition occurred at deep shelfal or bathyal water depths in the studied sections.

Age dating is based mainly on nannofossils with peak abundances equating with maximum flooding surfaces or lowstand condensed sections. Sequence boundaries are interpreted at 1.4 Ma (Early Pleistocene), 1.9 Ma, 2.4 Ma, 2.6 Ma and 3.0 Ma (Late Pliocene).

The palynological records from the Amazon Fan provide an excellent basis for well correlation and show that vegetation changes that have taken place within the Amazon catchment. The major control is likely to be climate.

Charred Gramineae (grass) cuticle is produced during times of fire in the savannah and "cerrado". The cuticle is found most commonly in sediments of Late Pliocene age, suggesting that this was a period of mostly dry climate and frequent fires. The highest abundances of cuticle may coincide with sea level lowstands and suggest the possible existence of  "El Nino" conditions in the pre-Quaternary.

Palynology is shown to be useful in determining types of river discharge in the palaeo-Amazon which can assist with sequence modelling and reservoir prediction. Deposition during periods dominated by "whitewater" discharge is predicted to be mainly mud-prone and is unlikely to result in deposition of extensive reservoir sands within the Amazon Fan. Deposition during periods of significant "clearwater" discharge is likely to be sand-prone and may produce potential reservoir intervals within the Amazon Fan.

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