Taphonomy and paleosynecology of the Lower Silurian graptoloid fauna, Cape Phillips Formation, Nunavut, Canada.

摘要

Uncompressed graptoloids from carbonate concretions of the Late Ordovician to Early Devonian Cape Phillips Formation, Arctic, Canada, yield valuable biostratinomic information unavailable in the study of flattened specimens. The concretions lithified during shallow burial before extensive physical compaction, preserving the three-dimensional nature of the graptoloids and the fine-grained sediment prior to dewatering (Melchin and Coniglio, 1995). Fifty-two concretions and two pieces of limestone beds from two sections on Cornwallis Island were dissolved layer-by-layer in acid, enabling the examination of the graptoloid preservational condition and distribution within single bedding planes and through successive strata at a high level of resolution.; Eleven micro lithofacies within the deep-water carbonate slope-to-apron facies in the Lower Silurian of the Cape Phillips Embayment are identified. I describe a process-based graptoloid taphofacies model and define seven distinct taphofacies that can be identified by comparing the microlithofacies with the graptoloid taphonomic characteristics including fragmentation, alignment, size sorting, species composition, and bedding plane concentration (assessed with chi-square). Four taphofacies describe graptoloid concentration beds and three taphofacies have a statistically random vertical distribution of graptoloids. The graptoloid concentration beds were the product of: increased input and concentration of graptoloid rhabdosomes by physical transport; increased input of graptoloids by a paleoecological bloom; cessation or reduction of sediment supply producing a heavily time-averaged graptoloid assemblage; or physical concentration by winnowing of sediment producing a sediment lag accumulation. Three taphofacies describe the non-concentrated graptoloid distribution: constant graptoloid - constant sediment input; sediment event but low graptoloid abundance; or bioturbation. All taphofacies with the exception of the bioturbation taphofacies were recorded from the concretionary material collected from the Cape Phillips Formation. The distribution of taphofacies may be useful, together with other information, in the reconstruction of sedimentary and sea-level history of the Cape Phillips Embayment.; Identification of background and episodic processes, together with measures of overall sediment thickness and estimates of section duration and stratigraphic completeness, permit the calculation of a range of possible, small-scale sedimentation rates for the concretionary strata. The time resolution of a 5 mm thick lamina of concretionary material (average sample size used in this thesis) is approximated to be between 25 and 114 years. The graptolites within that layer are time-averaged because the accumulation time is longer than the probable life span of a graptoloid colony. However, graptoloid assemblages were relatively stable and a cluster analysis identified graptolite assemblages to have an average stratigraphic thickness of 6.5 cm in the concretionary material, or 1.3 cm of shale (corrected for compaction).; Recurrent graptoloid species associations, identified by cluster analysis, are considered to represent graptoloid communities. Whereas some of these community changes are the result of evolutionary changes, others appear to be caused by local environmental changes, ecological blooms, or physical mixing through sediment transport. The concomitant distributions of rhabdosomes and siculae and observations of sicular orientation within the concretions were used as evidence to support the hypothesis that the mode of life of siculae was planktonic, like the mature rhabdosomes.