TECTONIC ORIGIN OF SHEARED BELEMNITES FROM CENTRAL ALASKA CONFIRMED BY FLUORESCENCE, CATHODOLUMINESCENCE AND TRACE ELEMENT MAPPING

Belemnites are extinct Mesozoic cephalopods characterized by an internal skeleton. Their finger-shaped rostra are common fossils in Jurassic-Cretaceous marine sediments. The centimetre-sized rostrum is composed of low Mg calcite which behaves brittle during deformation, fragmenting the original rostrum. Stretched rostra from belemnites found in shales have been used to determine extension parallel to the bedding plane. In contrast, numerous belemnite rostra of the species Cylindroteuthis, found in Lower Cretaceous calcareous mudstones of the eastern Talkeetna Mountains in central Alaska, are known to be sheared or bent, kept cohesive by fibrous calcite veins filling the fractures. Microstructural studies revealed several vein growth phases suggesting continuous deformation. The thin and mechanically weak mudstone is sandwiched between stronger limestone and sandstone, suggesting that deformation of the belemnite rostra is the result of tectonic activity along a thrust fault. However, deformed rostra could also result from injuries or diseases suffered by the animal. The biogenic calcite produced by the animal can be sufficiently differentiated from the abiogenic calcite that cemented the tectonically fractured rostra. Petrographic and geochemical methods, using transmitted light microscope with fluorescence and cathodoluminescence microscope and electron microprobe were applied. Biogenic calcite phases are characterized by radial fibrous calcite crystals with concentric growth lines. Growth lines fluoresce green-yellow and the rostra minerals show no luminescence in the cathodoluminescence analysis. On the other hand, the observed calcite within the fractures is lacking fluorescence and growth rings, and shows a strong red-orange luminescence in the cathodoluminescence, characteristic of Mn-rich abiogenic calcite phases. Similarly, electron microprobe reveals increased manganese and iron content in the fracture calcite, corroborating an abiogenic origin. In contrast, the biogenic calcite is enriched in magnesium and strontium compared to the abiogenic calcite. These results support the initial interpretation derived from macroscopic observations that the deformation of the belemnites of the Alaskan belemnites is the result of compressive stress during thrust faulting.