The lowermost Cambrian in the Valdelacasa Anticline (central Spain): some new palaeontological data  

GÁMEZ VINTANED, J. A. 1, PALACIOS, T., FERNÁNDEZ-REMOLAR, D. AND LINÁN, E.  

Successions straddling the Precambrian/Cambrian boundary in northern and southern Spain show important hiatuses, but this transition is recorded without important breaks in the central part of the country. The western part of the northern side of the Valdelacasa anticline (Toledo Mountains) is one of the most suitable areas to study the palaeontology of this transition in central Spain, and also one of the best in western Europe. Here, fine siliciclastics of the Río Huso Group (Pusa shale) crop out providing a fairly rich record of ichnofossils, small shelly fossils, trilobites and other groups. The stratigraphic succession is ca. 2,000 m thick and includes a lower unit of greenish shales, a middle unit including black, micro laminated shales, phosphate beds and conglomerates, and an upper unit of greenish shales, very fine sandstones and scarce calcareous sandstones.

Cambrian-diagnostic trace fossils appear from the base of the lower stratigraphic unit, including Phycodes pedum, Monomorphichnus lineatus and small specimens of Psammichnites ichnosp.

The middle unit contains phosphatized fossils, namely Cloudina, anabaritids, halkieriids, sponges and probably small arthropods.

The upper unit provided rich and diverse body fossils assemblages at several horizons, as well as abundant ichnofossils. A lower assemblage consists of small shelly fossils (aff. Aldanella, hyoliths and others, including forms of centimetric sizes) and trilobites of the Family Bigotinidae. Another assemblage, placed a few meters above in the column, contains more diverse small shelly fossils: monoplacophorans (aff. Aldanella), hyoliths (hyolithids, circothecids and orthothecids), possible protoconodonts (aff. Mongolitubulus) and coeloscleritophorans (chancelloriids). A recently found uppermost assemblage, placed about hundred meters above in the column and shortly below the base of the Azorejo Formation, has provided very abundant bigotinid trilobites, small shelly fossils (molluscs and others, including forms of centimetric sizes), trace fossils (Sericichnus and Teichichnus) and probably marticulate brachiopods.

Trace fossils in the upper unit of the Pusa shale are, as a whole, of bigger size than in underlying units and include feeding burrows of several patterns, such as Dactyloidites ichnosp., Treptichnus bifurcus and big specimens of Psammichnites gigas. This succession is overlain by sandstones and shales of the Azorejo Formation containing Rusophycus ichnosp.

According to its palaeontological record, the age of the fine siliciclastics of the Río Huso Group (Pusa shale) in the studied sections ranges from the lowermost Cambrian (Nemakit-Daldynian) to the latest Tommotian or Atdabanian.

Acknowledgements. Authors at Badajoz profited financial support from project PB98-0994 of the Spanish Ministerio de Educación y Cultura, and project IPR99C029, of the Consejería de Educación, Ciencia y Tecnología,Junta de Extremadura. Author at Zaragoza acknowledge financial support from project PB96-0744 of the Spanish Ministerio de Educación y Cultura.