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Biosequencing
Biostratigraphy

 

Biosequencing

The Biosequential concept is a comprehensive palynological facies model which allows the detailed description of palynological assemblages with respect to the deposition of sequences which has not yet been published. The status of paleoecological papers with respect to dinoflagellate cyst distributions was reviewed by Davies, Bujak & Williams (1982), but little progress has been made since. The preliminary ideas were presented by Davies & Bujak (1987) from work on the Plio-Pleistocene of northern Gulf of Mexico which demonstrated palynological cyclicity responding to glacial-interglacial climatic cycles of North America. The concept of a biosequence, defined as:

"The stratigraphic interval delimited by the development and deterioration of a fossil biome or 'paleobiome' represented by a series of fossil assemblages in succession."

 

In-house work has linked this cyclicity to sedimentological sequences where one palynological cycle or biosequence generally corresponds to a sedimentological sequence. This observation has lead to the development of an analytical technique that extracts this relationship into a biosequence log.

This technique described by Davies and Bujak (1987) initially on the Plio-Pliestocene of Gulf of Mexico offshore from Texas and Louisiana. It has been adapted and applied successfully to the Plio-Pleistocene of the Canadian Beaufort Sea, Oligocene to Miocene of the Gulf Coastal Plain, the Miocene to Pliocene of offshore British Columbia, the Barremian to Albian of the Western Interior of Canada and the Scotian Shelf of offshore eastern Canada, and the Latest Jurassic to Albian of the Mackenzie Delta.

A cyclicity in the occurrences and abundances of all groups of palynomorphs is evident in sedimentary sections and is related to biome expansion and collapse. The expanding boreal forest and changing tundra components coupled with sea level changes and variations in freshwater influx allows the discernment of sequential palynological assemblages. Similar cycles are present in Plio-Pleistocene wells from the northern Gulf of Mexico (Davies and Bujak 1987) and have been associated with variations in solar radiation accompanying cyclicity of the earth's orbit. The oscillating sealevel changes introduced by tectonic activity also are reflected in the variation in both terrestrial and marine floral realms.

In the Late Jurassic to Early Cretaceous of offshore eastern Canada, the principle driving forces for this palynomorph cyclicity in not known specifically. For simplicity it is assumed that the sequences expressed by the palynological assemblages represent eustatic sealevel changes associated with transgressive and regressive events. However the floral changes, which should reflect climatic cycles, parallel transgressive and regressive events, usually terminate at sequence boundaries.

The following sequence model is primarily based on the varying diversities and abundances of dinoflagellate cysts and secondarily on that of miospores. The assignment of samples to the sequences is accomplished by the empirical recognition of recurring palynomorph assemblage successions.

Initially, dinoflagellate and miospore diversities were calculated to delimit the major cyclical trends. The abundances of various palynomorph groups in each sample were also plotted in order to observe the general group successions.

The model for the idealized palynological biosequence for a complete development of a microfloral cycle has been constructed in order to express the changes in the palynological assemblages in terms of floral development and climate (warmth and precipitation) during the time of deposition. The succession of marine and freshwater algal communities and of the terrestrial floral communities through an idealised transgression regression sequence can be expressed by a palynological biosequence divided into four phases and ten assemblages

 

The four main driving forces for this cyclicity are:

  1. sediment influx (turbidity)
  2. freshwater influx (including salinity and nutrients)
  3. temperature (both air and sea)
  4. eustatic sea-level changes (the development of niches).

It is difficult however to discern the independent response to these forces without integrating with parallel sedimentological and foraminiferal studies.

 

THE BIOSEQUENCE CURVE

The climatic assemblage log indicates the changes in palynological assemblages with respect the idealized palynological biosequence. A full biosequence going through a glacial-interglacial-glacial cycle proceeds upward from left to right, with the next cycle subsequently starting from the far left.

 

Cool dry interglacial periods will miss some or all of the assemblage categories in Group 1 and 4. Hiatuses are reflected by incomplete cycles where optimum conditions indicated by assemblages within Groups 2 and 3 are subsequently followed by cooler 1A-1C assemblages. Other sequences may indicate hiatus by abrupt changes in assemblage sequences which do not conform to the idealized glacial-interglacial-glacial model.

 

 

This is a two dimensional representation of a helix traveling in time or depth through space of precipitation and temperature (Figure 6). The phases of the biosequence can be related to the sedimentological cyclicity expressed in sequence stratigraphic models.

 

The normal biosequence profile would best fit the maximum thickness in a typical systems tract slug model near the edge of the shelf break.

 

Throughout the sedimentary wedge of the sequence the biosequence patterns would vary predicably. A lowstand wedge will have a considerably different profile to that of highstand wedge.

Through utilizing the biosequence patterns linked into other geological and seismic analysis can provide key control to the basinal interpretation.

 

© Branta Biostratigraphy Ltd.
Edward H. Davies Ph.D., P.Geol.