THE MIDDLE.UPPER CAMBRIAN TRANSITION IN THE IBERIAN CHAINS. NE SPAIN. AN INTEGRATED APPROACH

Abstract

Dominantly siliciclastic, shallow-marine sediments of the Middle-Upper Cambrian transition in the Iberian Chains (NE Spain) accumulated through four main processes: the storm effects in open-sea conditions, the action of waves and currents in shoal com- plexes, the deposition from normal tidal currents in back-shoal areas, and the influence of synsedimentary tectonic processes. Founh-order shallowing-upward parasequences seem to represent episodes of pro- grading barrier island or delta trends, covered at the top by tidal-in- duced deposits. Superimposed shoner duration synsedimentary iecto- nic events took place, developing gravity-induced processes across lo- cal slopes within the basin. Trace fossil assemblages are broadly correlative with lithofacies and depositional sequences, which permit to reflect valuable informa- tions about benthic soft-bodied communities. They show a composr- te stratigraphic distribution, similar to the hierarchical scheme of se- quences. lntroduction. This paper shows an integrated approach to recon- struct benthic conditions in a siliciclastic platform, con- trolling the occurrence, distribution and preservation of body and trace fossils. Terrigenous sediments of the Middle-Upper Cambrian transition in the Iberian Chains ${E Spain) provide a good opportunity to eva- luate the paleoecological effects on a poorly fossiliferous seafloor of some significant geological markers: tran- sgressive-regressive depositional trends and their sequen- ce framework. Commonly, a pure lithofacial approach does not permit a complete interpretation of depositio- nal environments in stratigraphic successions. In this case, contribution of trace fossils is double: (1) many of the traces are facies-restricted serving as reliable indica- tors of different sedimentary processes within a broad depositional setting, and Q) yielding important environ- mental information in sediments characterized by scar- city of body fossils and physical sedimentary structures. Furthermore, isolated bioclastic concentrations will re- flect valuable information about episodes of faunistic de- velopment and sedimentary dynamics of the seafloor. The best approach in reconstructing the sedimen- i,ary aid benthic paleoecological conditions in a poorly fossiliferous siliciclastic seafloor is a complete facies and sequential analysis, incorporating evidence from body fossils and their taphonomic features, trace fossils and inorganic sedimentary structures.