GEOSCAN, résultats de la recherche


TitreThe problem of palynomorph recycling approached from an analysis of the assemblage - does it tell a story in spite of recycling?
AuteurWhite, J M
SourceNova Scotia 2010, Joint meeting of AASP-The Palynological Society (43rd Annual Meeting), the Canadian Association of Palynologists, and the Geological Association of Canada Paleontology AASP-The Palynological Society (43rd Annual Meeting), Geological Association of Canada Paleontology Division (20th Canadian Paleontology Conference) and Canadian Association of Palynologists, Program and Abstracts; 2010 p. 76
Séries alt.Secteur des sciences de la Terre, Contribution externe 20130258
ÉditeurAASP - The Palynological Society
RéunionNova Scotia 2010 (Joint meeting of AASP/CPC/CAP); Halifax; CA; Septembre 29 - Octobre 2, 2010
Sujetspalynomorphes; biostratigraphie; fossiles; Foraminifère; paléontologie; stratigraphie; Crétacé; Cénozoïque
ProgrammeCorridor et delta du Mackenzie, GEM : La géocartographie de l'énergie et des minéraux
Résumé(disponible en anglais seulement)
Recycling of palynomorphs is a classic biostratigraphic problem. Species out of stratigraphic context, or specimens of higher thermal maturity are easily detected but are likely a small portion of the assemblage, leaving much of the material undiagnosed as to source. Moreover, if recycling is from strata not much older, with no thermal maturity difference, or is largely long-ranging taxa such as Taxodiaceae-Cupressaceae-Taxaceae, how do you know if you have an assemblage that is indigenous and worthy of interpretation, or one to be ignored? There is no perfect answer, but the following approach was used for the Mallik gas hydrate research wells in the Beaufort-Mackenzie Basin (BMB).
Mallik 2L- and 5L-38 wells were drilled with chilled mud, partially cored and sampled at 5 m cutting intervals, and so merited quantitative analysis to refine Late Paleogene and Neogene palynostratigraphy. A difference in age determination between foraminifera and palynomorphs required a careful consideration of potential significant recycling.
Upper Cenozoic assemblages in the region contain abundant recycled palynomorphs, but given high latitude Tertiary forests, there must also be a useful palynological record. The climatic cooling trend since the middle Cenozoic should be expressed in the palynological assemblage. The evaluation of recycling was based on the premise that an assemblage reflecting evolutionary or environmental-driven change will be less random than one with a large component of recycled palynomorphs.
Various techniques were used to assess patterns of distribution in rare and common taxa. Concentration of palynomorphs per gram of cutting sample yielded consistent stratigraphic patterns. A percentage comparison of definitely Cretaceous taxa vs. potentially Cretaceous Taxodiaceae-Cupresseaceae-Taxaceae revealed diverging patterns. A One-sample Runs Test of stratigraphic clustering of rare taxa showed significant non-random patterns. It was concluded that assemblage was non-randomly patterned and interpretable, even though the recycled/in place status of individual specimens was unclear
Another insight into recycling came from a comparison of preparation techniques. All initial samples were unoxidized and considered kerogen. Samples below 445 m yielded residue on +180 µm screen. This residue was oxidized and scanned, yielding 2 analyses per sample. Correspondence analysis of taxa and samples yielded an ordination that separated the +180 µm, oxidized assemblage and the kerogen assemblage, with apparently more recycled taxa in the +180 µm fraction. Different assemblages from two organic fractions of the same sample cautions that sample preparation may skew the interpretation. Over what strata in the BMB this separation applies has not been defined.
Patterns of recycling identified increased erosion in the Late Eocene and the Miocene and also during Plio-Pleistocene glaciation. Analysis of coal cores argues that relict species may occur in paludal environments.
Résumé(Résumé en langage clair et simple, non publié)
On trouve des assemblages sporopolliniques dans des roches sédimentaires et ces assemblages représentent à la fois l'état de l'évolution et le climat qui existaient au moment de leur dépôt. Ainsi, les assemblages provenant de déblais de forage de puits de pétrole et de gaz peuvent servir à déterminer l'âge des roches qui ont été recoupées par un puits. Cependant, les roches peuvent contenir des assemblages qui proviennent de deux sources : la végétation qui croissait à proximité du lieu de dépôt et le pollen et les spores entraînés par l'érosion de roches plus anciennes. Cela crée une confusion dans l'interprétation de l'âge de roches renfermant des hydrocarbures dans le delta du Mackenzie et la mer de Beaufort. La présente étude utilise des analyses statistiques afin de déterminer quelle partie de l'assemblage sporopollinique provient de l'érosion de roches plus anciennes et quelle partie peut servir à une interprétation pertinente de l'âge des roches dans lesquelles se trouve cet assemblage.