Paleoenvironments and ichnotaxonomy of insect trace fossils in continental mudflat deposits of the Miocene Calatayud–Daroca Basin, Zaragoza, Spain

Abstract

The Miocene continental deposits in the Orera area, Calatayud–Daroca Basin, are composed of red and brown mudstones deposited in the dry mudflat, sepiolite and dolomicrites deposited in periodically desiccated alkaline shallow lakes by direct precipitation, and mudstones with carbonate laminae formed in wet mudflat areas by groundwater cementation during rise of water table. The mudstone/sepiolite + dolomite sequences represent dry–wet cycles. Two different ichnospecies of insect trace fossils were found: Fictovichnus gobiensis and Fictovichnus aragon n. isp. Fictovichnus gobiensis, are 15–18 mm long and 8–9 mm wide ellipsoid casts showing smooth surface. One end is rounded, whereas a truncated tunnel protrudes from the other. They have no discrete wall but some show an outer micritic layer. One end of Fictovichnus aragon n. isp. is rounded and the other pointed. The holotype is 26 mm long and 9 mm wide. In the contact with the matrix specimens show two micritic layers, interpreted as remains of the original silky wall constructed by the wasp larva. In both ichnospecies the micromorphology of the casts is similar to the matrix where they are found. Possible producers of Fictovichnus gobiensis are coleopterans, whereas Fictovichnus aragon was probably produced by sphecid or pompilid wasps. Indicative of subaerial exposition, Fictovichnus gobiensis is found in all four facies/environments. Occurrence of Fictovichnus aragon n. isp., restricted to the wet mudflat, is controlled by: a) shallow groundwater that permits the growth of phreatophytic plants but also the nesting of wasps in dry upper soil horizons, and b) the rise of groundwater table that favored decomposition of plants and cementation and preservation of the traces. In the study area the sedimentary environments controlled the location of the different traces, and diagenetic processes were crucial for their preservation. The lack of early diagenetic processes may explain the rarity of insect trace fossils in other similar continental basins.