Boletín de la RSEHN. Sección Geológica

«Lo Hueco» ha proporcionado abundantes y diversos fósiles del Campaniense superior-Maastrichtiense inferior correspondientes, mayoritariamente a vertebrados, aunque también a plantas e invertebrados. El yacimiento se encuentra en un tramo en facies ´´Garumn´´ de la Formación Villalba de la Sierra. En él, se distingue una sucesión de arcillas margosas versicolores (V, G1, R1, G2, R2 y M) separadas por límites graduales. Esta sucesión resulta parcialmente modificada por un primer intervalo sulfatado (S1), que corta el nivel V mediante un límite neto; por una estructura canaliforme (C), que corta los niveles V, G1 y R1 por medio de una superficie erosiva; y por un segundo intervalo sulfatado (S2), que distorsiona, al menos, el nivel G2 mediante un límite neto. La práctica totalidad de los fósiles obtenidos proceden de cuatro conjuntos estratigráficos (estructura C, niveles G1 y G2, y parte inferior del nivel R2) que pueden considerarse unidades de acumulación y definirse como tafofacies, atendiendo a sus particularidades tafonómicas y geológicas. La estructura C mayoritariamente contiene macrorrestos de vertebrados con elevada alteración bioestratinómica (superficies de abrasión y elementos desarticulados y fragmentados) y distribución aleatoria. Por el contrario, las arcillas margosas grises (en especial el nivel G1) principalmente muestran macrorrestos con escasa alteración bioestratinómica y elementos asociados, parcialmente articulados o constituyendo esqueletos casi completos. La mayoría de los macrorrestos de vertebrados presentan: una primera precipitación de yeso (interna), una costra ferruginosa (superficial), y una segunda precipitación de yeso (externa, aunque puede atravesar la costra y la primera precipitación). Además, los macrorrestos óseos de la estructura C y el nivel G1 por lo general microscópicamente muestran microfracturas radiales y anillos ferruginosos (interiores). La excepcional riqueza y calidad de este yacimiento parece haberse concentrado en una llanura de inundación continental fangosa, atravesada por canales distribuidores arenosos, y expuesta intermitentemente a aguas dulces, salobres o marinas.

The palaeontological site of «Lo Hueco» (Cuenca, Spain) contains an extraordinarily rich and varied fossil assemblage assigned to the upper Campanian-lower Maastrichtian (Upper Cretaceous), mainly composed by vertebrates, but also by plants and invertebrates, and preserved in an stratigraphic interval in ´´Garumn´´ facies (informal term for marls, clays and gypsums, mainly of reddish coloration, deposited in shallow marine, coastal or continental environments of south-western Europe during Latest Cretaceous and Early Palaeogene) belonging to the upper part of the Villalba de la Sierra Formation. In ascending stratigraphic order, a succession of green (V), grey (G1), red (R1), grey (G2), red (R2) and brown (M) levels of marly mudstones separated by gradual boundaries can be identified in the site. This succession becomes partially modified in the eastern and southern area of the outcrop by a first sulphated interval (S1) that cuts the V level by means of a net boundary, in the southern area of the outcrop by a sandy channel structure (C) that interrupts the V, G1 and R1 levels by means of an erosive surface, and in the north-eastern area of the outcrop by a second sulphated interval (S2) that distorts at least a part of the G2 level by means of a net boundary. Nearly all the macrofossil remains are concentrated in four stratigraphic lithosomes (C structure, G1 and G2 levels and lower part of the R2 level) that can be considered as accumulation units and defined as taphofacies, in view of their particular taphonomical and geological characteristics. Vertebrate macrofossils show different preservational patterns in which specific biostratinomic and fossildiagenetic pathways can be differentiated. C structure contains the major number of highly altered (with evidences of abrasion and fragmentation) and randomly distributed macrofossils. By contrast, grey marly mudstone levels (especially G1) preserve the major number of totally or partially articulated specimens (mainly of sauropods), with a relatively low degree of bone scattering. Most of vertebrate macroremains present from inside to outside: a first infilling of gypsum (reaching internal natural cavities), a ferruginous crust (covering the remains), and a second precipitation of gypsum (eventually passing through the ferruginous crust and the first infilling of gypsum). Additionally, most of vertebrate bony macroremains from the C structure and the G1 level microscopically show radial microcracks and ferruginous rings (all of them inside the bones). The exceptional richness and quality of the fossil concentration of this site seems to have been accumulated in a specific sedimentary environment (near coast continental muddy flood plain crossed by distributary sandy channels, exposed to intermittent fresh, brackish or marine water).