Palaeozoic stromatoporoid taphonomy: ecologic and environmental significance

Abstract

Epibenthic sessile marine organisms, such as the aspiculate stromatoporoids, show a wide range of growth forms in different settings across Palaeozoic carbonate banks and reefs, making them useful taphonomic tools in palaeoenvironment analysis. Field observations of Ordovician through Late Devonian stromatoporoid-bearing biostromal and biohermal successions on different palaeocontinents have revealed three main post-mortem tapho-signatures that affect palaeoecologic analysis: (1) variability of physical breakage and bioerosion prior to burial; (2) loss of morphologic and taxonomic information due to post-burial diagenesis; and (3) minor localized, reorientation during karstification. Physical fragmentation, abrasion, and lesser bioerosion and dissolution represent the main processes responsible for post-mortem destruction. Hydrodynamic behaviour of stromatoporoid clasts was similar to other carbonate skeletal clasts in their rounding and sorting characteristics. Although bioerosion and bioturbation did not play an important role in Palaeozoic stromatoporoid taphofacies characterization, a marked increase in bioerosion of stromatoporoids is evident, beginning in certain Silurian (Wenlock) morphotypes and in some Devonian morphotypes. Stromatoporoids offer varying profiles to shelf currents and seasonal storm surges, and apparently have different skeletal strength (durability) depending on skeletal architecture. Cyst-dominated skeleton architecture appears to have been fairly resistant to fragmentation. Latilaminae, a common skeletal feature in different families of calcified sponges, represent inherent zones of weakness that appear to have enhanced equidimensional (breakage away from latilaminae) to oblate (breakage along latilaminar surfaces) fragmentation. Construction of lamina-pillar architecture, amongst the most common skeletal attributes of Siluro–Devonian stromatoporoids, gives the visual impression of varying robustness, but this is supported by few data. Stromatoporoid response to increasing hydrodynamic energies varies greatly from no movement or simple overturning of skeletons to severe fragmentation and size sorting. Although such variation broadly reflects palaeogeographic setting and thus periodic disturbance by tropical storms, familial and generic skeletal traits greatly influenced taphofacies characterization. These differences in modularity and ontogeny in the stromatoporoid groups has hampered development of generalized taphofacies models, as devised for shell-rich deposits. Taphofacies are not comparable between modern coralgal reefs, especially the highly specialized acroporid facies, and Palaeozoic stromatoporoid-bearing reefs for the following reasons: significant differences exist in diversity of observable growth strategies (nature of attachment of skeletons to substrate and variety of morphotypes); probable significant differences in life modes between these groups (feeding strategies and positions and areal extent of clonal tissue within the skeletons, and their subsequent susceptibility to infestation by microborers); and, Palaeozoic macrobioeroders were fewer than modern representatives and apparently less effective in stromatoporoid taphofacies characterization than modern reef macrobioeroders are for coral taphofacies development.