Abstract:

Although major Late Ordovician glaciation was confined to the terminal Ordovician Hirnantian Stage, lasting less than 2 million years, recent isotopic evidence suggests a prolonged interval of slow growth in polar ice caps. Tropical carbonate platforms from passive margins and epicontinental seas offer the opportunity to monitor changes in eustatic sea-level changes that accompanied this transition from a greenhouse towards an icehouse climate. The Late Ordovician Bighorn Dolomite of northern Wyoming and adjacent states preserves such a transition. The Bighorn Dolomite was deposited on a northeastward-facing, subequatorial platform, and it consists of three third-order depositional sequences. Each is bounded by a regional erosional surface, commonly with erosional truncation visible at outcrop scale and commonly mantled by a distinctive brecciated horizon. The Bighorn Dolomite is dominated by highly bioturbated and fossiliferous open marine shallow subtidal facies, but portions also contain dolomitic bioturbated restricted marine shallow subtidal and laminated peritidal facies, as well as thin-bedded argillaceous and fossiliferous deep subtidal limestone. The three depositional sequences preserve a transition in their internal cyclicity that is consistent with computer modeling of the stratigraphic signal of increasing period and amplitude of eustatic sea-level changes. The lowermost sequence, which spans the Lander and Steamboat Point members, consists of homogeneous, acyclic, shallow subtidal facies, with a few meter-scale peritidal parasequences near its top. The middle sequence, which corresponds to the Leigh Member, contains acyclic to weakly cyclic restricted shallow subtidal and peritidal facies. The uppermost sequence, consisting of the Horseshoe Mountain Member, is dominated by five decameter-scale parasequences composed of open marine shallow subtidal, restricted shallow subtidal, and peritidal facies. Overall, the Bighorn Dolomite is interpreted to represent deposition during a shift from low-amplitude (a few meters), short-period (∼ 20 kyr) cyclicity to moderate-amplitude (∼ 10 m), long-period (100 kyr) cyclicity indicating a shift from greenhouse conditions towards icehouse conditions, a shift that agrees with recent isotopic evidence for changing Late Ordovician climate.

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