3) Miocene Palynology of the Nam Con Son Basin, Offshore Vietnam

KEITH RICHARDS, STEPHEN LOWE and MIKE BIDGOOD

POSTER ABSTRACT: Presented at Tenth International Palynological Congress, Nanjing, China (June 2000) and at Geological Society of America Meeting, Reno, USA (November 2000).

PALYNOLOGY AND SEQUENCE STRATIGRAPHY OF A DEEP WATER WELL SECTION OF MIOCENE AGE FROM THE SOUTH EAST NAM CON SON BASIN, OFFSHORE VIETNAM

Palynological data are presented from an interval of Middle Miocene age, in a well drilled in the South East Nam Con Son Basin, offshore Vietnam. The studied section forms part of the Mang Cau and Thong Formations and falls within calcareous nannofossil zones NN7 to NN5 and planktonic foraminiferal zones N13 to N8. Quantitative palynological analyses were carried out on ditch cuttings and sidewall core samples at 3m to 6m intervals over a section of approximately 230m in thickness. The studied section can be divided into 3 principal seismic sequences but these can be further sub-divided on the basis of palynology and log character into seven palynological zones, all of which coincide with significant seismic reflectors and changes in log signature.

Palynological zones I, III,V and VII are characterised by relatively high abundance and diversity of marine microplankton (mostly dinocysts and microforaminiferal test linings) and were deposited in marine, mostly outer shelf to upper bathyal settings. Palynological zones II, IV and VI contain lower proportions of marine elements and increased relative abundances of terrestrially derived elements including spores, bisaccate pollen and freshwater algae. Deposition occurred in a marine, mainly middle or outer shelf setting.

A sequence stratigraphic interpretation is presented which is based primarily on palynological and log data. Peaks of marine microplankton coincide with high gamma ray responses and represent high base level events (mostly within transgressive systems tracts). Intervening sections are characterised by lower abundance and diversity of marine elements, and coincide with lower gamma profiles (usually upward coarsening). These relate to low base level events and represent regressive depositional trends (mostly within highstand systems tracts). Palynological zone IV contains relatively common reworked forms indicating incision of older (Mesozoic) sediments during a period of relative lowstand (lowstand systems tract). The interpreted sequences are likely to equate broadly with third or fourth order depositional cycles.

The sequence model derived from palynological and log data is consistent with seismic data in that interpreted sequence boundaries and the principal marine flooding events coincide with regionally correlatable seismic reflectors. The data demonstrate that palynology is an extremely valuable tool for sequence interpretation, even in relatively deep marine environments such as those occurring the South East Nam Con Son during the Miocene.

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