FACTORS CONTROLLING UPPER JURASSIC AMMONITE ASSEMBLAGES IN NORTH CENTRAL MEXICO

Lower Kimmeridgian to Lower Tithonian (Upper Jurassic) sections studie d at Sierra de Palotes (Durango) and Sierra de Catorce (San Luis Potos i), Mexico, show low-energ deposits in which the composition of fossil macroinvertebrate assemblages, including megabenthos, reflects biostr atinomic control. Monotonous siltstones provide continuous records of ammonite assemblages and reflect dominant deposition of shells in livi ng areas; meanwhile, discontinues records were forced by episodic post -mort em transportation of shells, which was especially accentuated un der storm influence. Rhythmic marly-silty limestones and marls illustr ate a fossil record probably determined by minor transgressive-regress ive pulses. The major changes in lithofacies are reflected by condense d silty and phosphatic mudstones deposited during significant flooding s affecting areas under dominant terrigenous sedimentation. These chan ges determined more or less significant variations in the composition of fossil assemblages according to their relation to changing ecologic al conditions. However, shifting ecospaces exhibit no direct relations hip to changes in lithofacies. Post-mortem transportation, operating i n relation to both marine floodings and changes in the pattern of uppe r-water currents, was the main biostratinomic factor affecting the are al distribution of ammonite populations. Shell transportation and sedi mentation rate controlled preservation and ultimately influenced diver sity in recorded ammonite assemblages. The post-mortem behaviour (inte rpreted from shell structure and preservation), and therefore distribu tion, of ammonite shells points to shallow-water environments during t he Kimmeridgian - Early Tithonian in areas (such as SE Durango and San Luis Potosi) close to the changing boundary between dominant carbonat e and terrigenous sedimentation. No reworking affecting ammonite biost ratigraphy has been identified in the sections studied.