The intertidal zone off a beach at Tachero on the northeast coast of Tenerife in the Canary Islands features large clasts (35 to 75 cm in diameter) composed of basalt and scoriaceous tuff crowned by the fossil remains of crustose coralline algae (CCA). Overall, the boulder field occupies an area 7 × 20 m parallel to the present-day shore. Part of the underlying shore platform exposes 1.5 m2 of basalt pavement covered with the patchy remnants of the same coralline algae. Accessible during low tide, most boulders are stabilized and held in place by their mass at an average estimated weight of 600 kg. Individual algal caps are largely preserved in an upright orientation. Algal rinds attain thicknesses from 3 to 20 cm that typically drape the sides of the largest boulders. Cut and polished crusts show variability in algal density between 35 and 80
Oncoids, which are a type of microbialite, are widely known from the Precambrian to the present. However, the types of earliest Palaeozoic calcimicrobes within oncoids and their roles in oncoid construction are poorly understood. This study focuses on the construction of Cambrian oncoids from western Mongolia, based on microstructural types (i.e., types A–D), the variety of calcimicrobes and the abundance of fenestral fabrics. Type A microstructure is characterized by well-developed laminae and is dominated by Girvanella. Type B is similar to type A, but with an increased abundance of Kordephyton and Botomaella and with some fenestral fabrics. Type C is dominated by fenestral fabrics. Type D is markedly different from the other types, being characterized by irregular growth forms. Microstructures of types A–C exhibit gradational transitions in their laminae, calcimicrobes and fenestral fabrics, representing a continuous series that formed under changing hydrodynamic conditions. Type A was periodically overturned, forming laminated microstructures. Type C experienced episodic overturning that facilitated the development of Kordephyton and Botomaella, which contributed to the formation of fenestral fabrics and hindered the development of concentric laminae. Type B reflects conditions intermediate between those of types A and C. In contrast, type D was formed during long periods of quiescence with episodic overturning, resulting in highly irregular forms. These early Cambrian oncoids of western Mongolia are recording various microstructures with diverse calcimicrobial communities. The studied oncoids thus provide new information on how the early diversified calcimicrobes contributed to the construction of oncoids in the unique marine environment of the early Cambrian.
Based on an integrated dataset, the biostratigraphy of the late Cenomanian-Turonian ammonites and benthic fauna from northern Egypt is evaluated. The Cenomanian-Turonian successions were measured from six stratigraphic sections in north Eastern Desert and Sinai, represented by the mixed siliciclastic-carbonate sediments, comprising intertidal, shallow subtidal, and deep subtidal facies associations. A quantitative biochronological analysis is performed on the dataset by means of the unitary association method (UAM), using the Past software. This led to the recognition of a sequence of 5 ammonite unitary associations (maximal sets of actually or virtually coexisting taxa). The ammonite taxonomic richness is also illustrated, with respect to the Oceanic Anoxic Event 2 (OAE2). The regional correlation using UAZ is more straightforward and presents fewer boundary contradictions than classical interval zones. Furthermore, the unitary association method was performed on the benthic fauna. Consequently, twelve benthic unitary associations were constructed, and merged into 5 more geographically reproducible unitary association zones.
Rat’s Nest Cave in western Alberta, Canada, has six moonmilk accumulations on cave walls varying from 2 to 3 m2 to 100 m2 in area with thicknesses of 1–2 mm to > 15 cm. The compositions of the moonmilk deposits differ, consisting of low-Mg calcite needle-fibre crystals or aggregates of tabular euhedral crystals of hydromagnesite, but commonly mixtures. The needle-fibre crystals of calcite-type moonmilk are draped by a meshwork of thread-like filaments of extracellular polymeric substances (EPS), commonly calcified, and similar to other documented moonmilk deposits. In contrast, hydromagnesite-type deposits lack these nanofibres. Controls on the needle-fibre crystal morphogenesis include the adsorption of EPS organic molecules, Mg2+ ions from groundwater seeping into the cave, and other environmental factors that constrained the growth of crystal faces oriented in the a-axis direction while enhancing elongation in the c-axis direction. This interference on the structure of the cell lattice resulted in the needle-fibre habit as a type of skeletal crystal characterised by smooth well-defined surfaces of length-long grooves forming 120° Bravais-Miller junctions between the ±a1 and ±a2 axes and the suppression of crystal faces in the direction of the ±a3 axis. Epitaxial overgrowths on the needle-fibre crystals record the cessation of organic molecules and Mg2+ ion adsorptions on the surfaces of the underlying skeletal crystals, permitting the development of dendritic arrays of rhombic crystallites. This alternative model for the origin of the longitudinal grooves on needle-fibre calcite crystals in moonmilk contrasts with biogenic hypotheses that attribute groove formation to the earliest nanoscale growth stage generated by two adjoined rod-shaped precursors developed within organic sleeves of bacteria or fungal hyphae.
The late Cambrian is known for global biogeochemical perturbations to the marine carbonate system whose timing, extent, and duration remain unresolved, highlighting the need for high-resolution facies analyses to assess the nature of carbonate deposition. We investigate the dominant processes contributing to carbonate deposition using lithologic, microfacies, petrographic point count, and stable isotope analyses at a previously undescribed section at Rendezvous Peak, western Wyoming. The lower Pilgrim Limestone of the Gallatin Group represents deposition in an offshore storm-dominated carbonate platform composed of flat pebble conglomerates and laminated glauconitic and micropeloidal carbonate mudstones. Facies shift to middle and platform edge packstone/grainstone units that include two microbialite horizons. The upper Pilgrim Limestone returns to offshore flat pebble conglomerates and laminated micropeloidal carbonate mudstones during the continued interpreted marine transgression. Point count data indicate consistently low skeletal contributions (2.9
