NEW QUANTITATIVE DESCRIPTORS (SHAPE AND MACROBORING) OF BIOGENIC NODULES: EXAMPLES FROM THE LESSER ANTILLES AND NEW ZEALAND

Abstract

Encrusting (Ei), Macroboring (Mi), and Dissymmetry (Di) indices are proposed as quantitative descriptors of biogenic nodules. (Ei) measures the amount of encrustation, (Mi) the contribution of boring traces affecting the internal structure of nodules, and (Di) the regularity of the biogenic accretion around the nucleus. The latter has been used to construct a classification scheme of possible shapes for encrustations. Raw data to calculate the indices were obtained from numerical treatments of digitized photographs of nodule cross-sections. The morphometric (Di) and taphonomic (Ei and Mi) indices have been calculated for carbonate nodules from subtidal temperate and tropical settings in New Zealand (Flat Point Beach) and in the Caribbean (St. Bartholomew Island), respectively. Results for nucleated rhodoliths collected from shallow high-energy settings in these two climatic settings show that their morphometric and taphonomic characters are not species-specific (Lithoporella/Mastophora rhodoliths from St. Barth, and Lithothamnion-Sporolithon rhodoliths from New Zealand), but depend instead on hydrodynamic conditions and on the original shape of nuclei. Acervulinid macroids sampled in deeper waters (28 m) off St. Barth are nucleus-free and have a Macroboring index (Mi) significantly higher than that of rhodoliths from shallower environments, due to discontinuous influence of waves and currents, and low sedimentation rates. The quantitative descriptors proposed here might: (1) complement the characterization of biogenic nodules in specific depositional environments; (2) aid in hydrodynamic and paleoenvironmental reconstructions of biogenic nodule-bearing deposits; and (3) constitute valuable tools in future comparative studies.