Rudist reef structure: Insights from orientation of hippuritids at l'Espà (Campanian, southern Pyrenees)

Abstract

Rudist were common Mesozoic reef builders, but rare examples exist to evaluate reef structure. The l'Espà locality (southern Pyrenees) is approached to study rudist reef structure by a quantitative assessment of the individuals' position (growing vs reworked) and its sedimentary context. This late Campanian reef is exposed along some 20 × 6 m outcrop. Builders are mainly Hippurites radiosus, although other rudists, such as Hippuritella lapeirousei, Hippuritella sp. and Mitrocaprina sp. are also present together with corals. The orientation of 325 specimens of mainly H. radiosus was plotted in stereographic and cartesian projections. Orientations and microfacies permit to differentiate 5 vertically stacked intervals (settings) along the outcrop: (1) Distal reef setting (reef-talus slope), with rudists reworked as large bunches of grouped specimens, with scarce erosion and preserving both valves articulated; (2) Halfway distal-proximal reef setting (close cluster reef), with abundant reworked, isolated, and flat-lying specimens. In this zone, endo-epibiont colonization on rudist shells is common, together with the presence of large (up to 1 m) branching and massive corals; (3) Proximal reef setting (frame/close cluster reef), where specimens are in life position; (4) A proximal back reef unit (spaced cluster reef) with few highly reworked specimens; (5) Distal back reef setting (very spaced cluster reef), with hardly any rudist fragments. This succession provides a reef tract model resembling that of most coral reefs and differs from smaller rudist reefs. The structure of the studied reef is well preserved as a result of high accommodation space related to thrust emplacement.