Abstract

Frequency modulation (FM) analysis of a sequence of basinal limestone/shale cycles from the Upper Pliensbachian Domaro Limestone, Lombardy Pre-Alps, northern Italy, reveals modulations with multiple low frequencies that closely resemble those of the theoretical equatorial, orbitally forced (precession-dominated) insolation. There is evidence for a systematic shift in these modulation frequencies that is consistent with a shorter Jurassic length of day. These results were used to estimate the incremental time scale along the Domaro succession, revealing a dual-frequency signal comparable to the precession. Separate analysis of the limestones and shales suggests that the limestones were deposited during times of decreasing or low northern spring equatorial insolation (NSEI), and the shales during times of increasing or high NSEI. We discuss two "end member" models that could link precession-forced insolation to the cyclic basinal sedimentation. The ambiguity of the source of carbonate influx into this Early Jurassic basinal depocenter led us to consider (1) an "allodapic mud model" in which precession-forced sea-level oscillations instituted a cyclic production of lime mud in shallow marine environments and subsequent transport into the adjacent basinal environments, and alternatively, (2) a "pelagic ooze model" in which precession-forced insolation directly forced pelagic productivity and influx into the basin.

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