Bioclastic Accumulations in the Western Margin of the Lower Triassic Montney Formation

• Author / Creator

  Sanders, Shelby C

• Two bioclastic intervals within the Early Triassic Montney Formation in British Columbia, Canada, were examined for sedimentological and paleontological evidence of their deposition. Twenty seven core were analyzed for sedimentological characteristics and then sampled for further paleontological and petrological analyses. The analyses allowed for the construction of three facies associations that describe the depositional characteristics of these two units. T¬hese two bioclastic intervals represent some of the first calcified body fossils preserved within the Western Canada Sedimentary Basin after the P-Tr boundary. Preserved calcified body fossils are rare within the Montney Formation (Zonneveld, 2011). Their presence within these sections and absence within the rest of the basin allow insight to the unstable conditions within the Peace River Embayment in the Early Triassic. The first of the two bioclastic horizons, the “Claraia Zone”, has been mistakenly interpreted as an offshore turbidite fan. Analysis of core has revealed that this unit is a bioclastic interval that shows lithological characteristics inconsistent with this interpretation. The “Claraia Zone” has been delineated and mapped using core data sets and wire line log characteristics. Core were logged and facies identified based on lithological characteristics of the units. Two facies associations were connected to this horizon. The first, the fan-derived bottom current modified deposits, consists of heterolithic very fine sandstone interbedded with fine bituminous siltstone. These beds contain lenticular to flaser bedding that has been found to be remarkably similar to bottom current deposits from the Pleistocene of the Gulf of Mexico. The second facies association consists of distal bottom current modified deposits wherein a biological signal was able to be preserved. The primary facies of this association is the laminated bioclastic siltstone which is also the primary facies connected to the “Claraia Zone”. The bioclasts within this zone showed evidence of transport and/or winnowing in the form of highly fragmented and disarticulated valves. Similar contacts and grading found between this facies and the heterolithic facies allowed them to be connected to the same depositional mechanism. Finally, evidence of the possibility of bottom current modification was compared with the criteria for identifying bottom current deposits. The small scale sedimentary analysis showed evidence in the form of the heterolithic facies and the winnowing or bioclasts within the laminated bioclastic siltstone facies. The medium scale was based on the deposit shape which was extrapolated to be parallel to paleoshoreline from analysis of cross sections and isopach maps. Large scale evidence was linked to the location of the basin at the western margin of Pangea in the Early Triassic which would have been subjected to strong westerly winds and storms. Therefore it was interpreted that the “Claraia Zone” was the result of wind derived bottom current deposition/modification. The second, the Altares Packstone Member, was identified as the earliest example of in situ biogenic carbonate development in northwestern North America after the P-Tr boundary. These packstone beds are late Smithian in age and occur just below the Smithian/Spathian boundary (SMSP) that is correlated across the Montney Formation. They are interpreted to be a series of biostromes that were exhumed and eroded several times throughout their development and make up the final facies association within this study. The bioclastic packstone units appear in small packages, commonly no thicker than a few centimeters in core, but these packages are repeated through the horizon, and often have thin, calcified beds on either side that grade into the thicker packstone. Articulated brachiopod, bivalve, and ammonoid shells were observed within thin-sections through the packstone, along with several examples of vertebrate material. The abundance of articulated valve material (particularly of Pectinoids) in the center of the deposit indicates that these deposits likely occurred in situ. Biostromes have been referred to being the result of Allogenic Taphonomic Feedback, but without the necessary evidence of hard ground preferring organisms that theory cannot be substantiated here. These biostrome beds are discrete features that are not correlatable across large distances. Therefore, although these beds contain low concentrations of bitumen and could act as a baffle to exploration, their discrete nature indicates that they are not a complete obstruction.

• Subjects / Keywords

  • Biostrome
  • Bioclastic
  • Bottom current
  • Montney Formation

• Graduation date

  Spring 2016

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3R78618X

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.