Catodoluminiscencia y diagénesis en conchas de rudistas (Bivalvia) del Cretácico Superior de Gredilla de Sedaño (Norte de Burgos, España)

Abstract

The Quintanaloma Fm (Campanian-Maastrichtian) was deposited in a shallow marine environment and contains a lot of rudist shells (Hippuritidae and Radiolitidae), some of them in life position. From a diagenetic point of view, the Radiolitidae seem to be the more interesting, because their cellular microstructure («honeycomb») is-propitious for cement precipitation and mineral authigenesis. Taking into consideration that a pristine composition of the bivalve shells implies the lack of activators such as Mn++, the presence of luminescence in shells can be explained in terms of organic matter diagenetic alteration. Cathodoluminescence (CL) demonstrates that the more obscure and organic-rich a part of the shell, the more luminescent is. Luminescence in radiolitid shells is more evident just in the transition from the upper part of the walls to the base of the subsequent growth line. This could be explained as a result of a vital effect, when the organism needs to reinforce the epitelial mantle, or increase the secretion of extra-paleal liquids. The diagenetic alteration degree was not enough to homogenize the luminescence of the whole shell. From microscopic observations, diagenesis in radiolitid shells developes at two levels:

1) cement precipitation (intense-red dolomite rhombs; well-zoned A cements, some of them «dogtooth»-type; and blotchy/irregularly-zoned blocky calcite B cement); and 2) two phases of recrystallization, which sharply and indiscriminately eliminate the luminiscence of both shells and previous cements