CONTRACTIONALLY DEFORMED BELEMNITES AS INDICATORS FOR THRUST FAULTING IN MECHANICALLY WEAK SEDIMENTARY LAYERS – AN EXAMPLE FROM THE LOWER CRETACEOUS OF THE TALKEETNA MTS., CENTRAL ALASKA

"Stretched belemnites" have long been used in tectonic studies to determine extension by measuring the elongation of their rod-shaped guards. Evidence for compression, on the other hand, has not yet been reported from belemnite fossils. In calcareous mudstone from the Lower Cretaceous of the Talkeetna Mts., central Alaska, belemnite rostra of the species Cylindroteuthis display signs of contractional deformation: shear fractures and bent guards. Fibrous calcite that grew in the fractures prevent disintegration of the deformed rostra. Several growth phases of calcite fibres indicate that deformation was not a single event. The sheared belemnites are found in a relative thin (10-15 m) layer of mechanically weak calcareous mudstone under- and overlain by thick and strong limestone and sandstone formations. Regional contractional tectonics resulted in thrust nappes developed in the limestone accommodating hundreds of meters shortening with the relative weak calcareous mudstone most likely acting as a gliding horizon. Unfortunately, the deformed belemnites are not found in situ — they are washed out by rain from the poorly weathering resistant mudstone — so that strain analysis is not possible. Though significantly mechanically weaker than the adjacent limestone and sandstone, the unweathered mudstone is competent enough to allow deformation of the belemnite guards without complete strain partitioning around them. Some of the belemnites develop shear fractures at various angles depending on their orientation with respect to the σ1 stress axis. The presence of contractional deformed belemnite guards thus can serve as an indicator of thrust faulting or general shortening in soft sedimentary rocks that do not display any internal deformation associated with compressive stress that is common in more competent rocks, such as duplex structures.