Petrography and chemical composition of the trace fossil ophiomorpha nodosa in Nangang formation, Northeast coast of Taiwan

Abstract

Trace fossils provide a geological record of biological activity and are able to indicate changes in the depositional environment. Trace fossils including a burrow system of Ophiomorpha generated by crustaceans are common in outcrops along the northeast coast of Taiwan. These trace fossils are often preferentially preserved in the outcrops, indicating a different mineral composition in comparison to host sediments. Therefore, this study investigated geochemical aspects of the differential diagenesis of trace fossils and surrounding host sediment. Several sandstone samples were collected showing preservation of the trace fossil in epirelief (i.e., more resistant than surrounding rock) and hyporelief (i.e., less resistant than surrounding rock), in the same sedimentary succession. Geochemical investigation was performed using petrographical thin-sections, Itrax XRF core scanning of rock slab samples that were cut perpendicular through Ophiomorpha nodosa burrows. Also, XRF and XRD are conventionally used for spot analysis of powdered-samples in order to observe chemical and mineralogical compositions respectively. Based on observations made in this study, samples of trace fossil preserved both in epirelief and hyporelief are mainly composed Iron (Fe) that found Goethite (FeO(OH)). Similarly, they have low contents of Silicon (Si), Aluminum (Al), and Potassium (K) that found Quartz (SiO2), Feldspar (KAlSiO2) and Illite-mica ((Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)]). The main difference between two types is petrography that the trace fossil in hyporelief were finer-grained and more matrix-supported compared to the trace fossil in epirelief under microscope. These observations lead to a preliminary interpretation for this study area that the preservation of Ophiomorpha nodosa is related to diagenetic mineralization of the burrow walls, causing differential erosion and weathering. Trace fossils that is grain-support and less matrix are often preferentially preserved, while trace fossils with more matrix-support are preferentially eroded.