Carrizo Arroyo, central New Mexico - a new late Palaeozoic taphotype of arthropod Fossillagerst�tte

At Carrizo Arroyo, southwest of Albuquerque, New Mexico, an approximately 100-m-thick section of the latest Pennsylvanian (latest Gzhelian) to Early Permian (early Asselian) Red Tanks Member of the Bursum Formation is exposed. This sedimentary succession is interpreted as a coastal plain on a very shallow shelf affected by repeated transgressions and regressions. Besides the marine marls and limestones, the most common lithotypes in the nonmarine fossiliferous intervals are greenish-gray and gray, variably sandy fine clastics. Lithology and facies architectures together document a low energy floodplain environment crossed by very shallow but wide flood channels. In the floodplain deposits, three basic taphotypes were observed: (1) the common plant bed type, (2) the rare conchostracan bed type, and (3) the insect bed type, which is not as rare as previously assumed. Plant beds are commonly formed by single layers of dm-long branches and leaves, as well as cm-sized plant fragments. Consequently, it is assumed that the Carrizo Arroyo plant beds were deposited by waning flood in shallow and wide floodplain channels. Conchostracan and insect beds have several features in common. Bedding planes with enrichments of conchostracans, freshwater pelecypods, insects, and, in places, eurypterids, contain tiny plant detritus of mmto cm-size only. They form a sub-mm to mm-thick layer only, and have a restricted lateral extent of several meters to decameters. Altogether, this points to autochthonous assemblages of aquatic arthropods and molluscs preserved in short-lived freshwater puddles and ponds on the floodplain. The common but generally isolated insect wings were most likely transported by winds and trapped at the water surface of those freshwater accumulations on the floodplain. Obviously, fossiliferous deposits at Carrizo Arroyo contain an assemblage of autochthonous and allochthonous elements of the insect fauna, covering environments from the hinterland down to the seacoast. This makes the Carrizo Arroyo Fossillagerstätte exceptional. 377