Application of Bryozoan zoarial growth-form studies in facies analysis of non-tropical carbonate deposits in New Zealand

Abstract

The fragmental remains of bryozoan colonies are the dominant skeletal contributor in many modern and ancient occurrences of non-tropical shelf carbonate deposits. The taxonomic complexity of the Bryozoa, the difficulties of systematic classification except by specialists, the wide range of particle sizes and of preservation state in the carbonate deposits, and the often well-cemented nature of host limestones, all limit the sedimentological interpretation of the commonly diverse bryozoan component. However, following the pioneer work of Stach in the 1930's, the potential exists for sedimentologists to obtain useful (paleo)environmental information about the deposits by recording simply the nature and relative abundances of the various growth shapes of the bryozoan material, known as their zoarial (colonial) growth forms. Here we propose a simple descriptive terminology, covering both cheilostome and cyclostome bryozoans, that involves four main growth forms (encrusting, erect rigid, erect flexible and free-living) and a selection of subcategories based on shape. Identification of these habitat-related growth-form types is aided by reference to line drawings, specimen photographs and photomicrographs of thin-section slices. The scheme is illustrated by examining briefly some possible environmental controls on the spatial and/or time variations in the nature and distribution of colonial growth forms in several examples of modern and Tertiary bryozoan-dominated carbonate deposits in New Zealand. We anticipate that the routine description and quantification of bryozoan colonial growth types in non-tropical carbonates generally will facilitate the recognition and interpretation of contrasting (sub)facies within the global foramol/bryomol group of carbonate deposits.