Polytopic Vector Analysis of Foraminiferal Faunas from the Offshore Yakataga Formation (Late Cenozoic-Gulf Of Alaska): A Potential Tool for Sequence Biostratigraphers

Sarah D. Zellers

Paleontologic samples often consist of fossil assemblages that are mixtures of fauna from different paleoenvironments and ages. These mixtures are usually the result of geologic processes (e.g. downslope transport of living or fossil material), but may also be due to problems in data collection (e.g. downhole contamination). These mixed assemblages can present problems when trying to use microfossil distribution patterns for identifying sequence boundaries or condensed sections (sequence biostratigraphy).

Polytopic Vector Analysis (PVA) is a multivariate quantitative ordination method that was designed to "unmix" geological samples, and has been used successfully in unmixing foraminiferal assemblages. Output from PVA includes end member compositions which reflect the importance of each species in defining each end member and deneg proportions which indicate the proportion of each end member in the sample. This output provides information on intrasample and intersample variability (deneg proportions) and can be used to identify components within the mixed assemblages which can lead to insight into the origin of mixing processes. Results of PVA data on benthic foraminiferal data from four petroleum wells in the offshore Yakataga Fm. in the northeastern Gulf of Alaska are presented to ill strate the use of PVA in a sequence biostratigraphic framework.

The Neogene through Quaternary Yakataga Fm. is a 7 km thick succession of interbedded glacial marine and normal marine clastics that is an excellent record of the interplay between tectonics, sedimentation, eustasy, and glaciation along an active continental margin. Eight tectonically controlled seismic stratigraphic sequences were recognized in the offshore Yakataga Fm. using foraminiferal sequence biostratigraphy and seismic stratigraphy. PVA was used to further investigate the nature of foraminiferal assemblages within the offshore wells in order to improve sequence biostratigraphic interpretations and to identify the processes yielding mixed assemblages along this margin.

The following seven end members, interpreted as biofacies, were identified in the Yakataga data set using PVA: EM1--Elphidium excavatum clavatum (inner neritic); EM2--Epistominella pacifica (upper bathyal); EM3--sharp margined Cassidulina; EM4--Cassidulina californica (outer neritic); EM5--round margined Cassidulina; EM6--Uvigerina peregrina (upper bathyal); and EM7--Haplophragmoides spp. (outer neritic). Most samples contain a mixture of two or more of these different biofacies. Geologic mixing on the Yakataga continental margin is probably due to the following: 1) downslope transport of living or fossils specimens due to slumping; 2) transport of faunas by icebergs or sea ice; 3) constriction of water masses due to sea l vel changes; 3) or juxtaposition of water masses due to position of glacial ice. Identification of these processes has yielded a better understanding of the development of stratigraphic sequences along this margin.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995