ABSTRACT This study provides a composite record of 87 Sr/ 86 Sr, δ 18 O, and δ 13 C for three sections in the Tethyan Lower Cretaceous Maiolica formation, a pelagic limestone from the Umbria-Marche Apennines of Italy, carefully tied to a magnetostratigraphically and biostratigraphically calibrated time scale. Although the 87 Sr/ 86 Sr record accurately follows the trend of the global marine 87 Sr/ 86 Sr reference curve, individual Sr isotope ratio values are relatively high for their inferred stratigraphic position, with all 87 Sr/ 86 Sr ratios yielding a fairly uniform +0.00007 to +0.0001 discrepancy. This offset likely results from incorporation of excess 87 Sr through isotopic reequilibration with interstitial pore waters during progressive lithification of the calcareous ooze. Although the process occurs principally through dissolution-reprecipitation, buffering the contemporaneous seawater Sr isotopic signature, diffusive communication with the overlying water column and porous sediments will compete with the dissolution-precipitation process, homogenizing pore-fluid concentrations and isotope ratios throughout the sediment column. Because the secular trend in 87 Sr/ 86 Sr throughout the Maiolica time frame is one of constant increase before rebounding to lower 87 Sr/ 86 Sr ratios in the Barremian, the ratios of the Maiolica carbonates are systematically displaced from that of the seawater in which they were deposited toward more radiogenic (higher 87 Sr/ 86 Sr) values. In addition, the carbon and oxygen isotope record of the Maiolica limestone allows identification of the mid-Valanginian Weissert event, characterized by a positive excursion in the δ 13 C and the δ 18 O records. Furthermore, the Weissert event correlates with a positive spike (+0.0001) in 87 Sr/ 86 Sr. Both the Sr and O isotopic peak signals predate the maximum peak in the δ 13 C excursion. This is likely a diagenetic artifact and may support the hypothesis of diffusive communication during lithification of the calcareous ooze.

ABSTRACT Breccias affecting the pelagic Lower Cretaceous Maiolica limestone of the Umbria-Marche Apennines of central Italy contain 10-cm-diameter to submillimeter angular clasts of white pelagic limestone and black chert, separated by a filling of sparry calcite. The clasts can often be seen to have originally fitted together, indicating extension without shear, and this is the case in all three dimensions, arguing for roughly isotropic volumetric expansion. Breccia fragments are separated by sparry calcite bodies comparable in width to the fragments; this shows that the breccias were not formed by collapse, or by a single large explosion, after either of which the fragments would surely have fallen to the bottom of the cavity, but probably by multiple small expansion events, each followed by calcite deposition in the small voids that opened up. The breccia sometimes occurs in dramatic topographic walls, a few tens of meters in both width and height, although there is not a one-to-one correspondence between breccia and walls. The sparry-calcite fill indicates that water with dissolved CO 2 was involved in formation of the breccias, presumably providing the high fluid pressure that forced the fragments apart. The breccia is bounded stratigraphically above by the middle Cretaceous Marne a Fucoidi (Fucoid marls), which appears to represent an aquiclude that limited the volume of high fluid pressure ( P F ). Although the mechanism of formation of the expansion breccias is not yet clear, we list observations that need to be accounted for by such a mechanism and discuss how these observations might be explained.