Geochronologic constraints on the chronostratigraphic framework of the Neoproterozoic Huqf Supergroup, Sultanate of Oman

Abstract

The Huqf Supergroup, Sultanate of Oman, contains an important record of Neoproterozoic history, including evidence for two glaciations, a massive reorganization of the global carbon cycle, and the Ediacaran-Cambrian transition. New U-Pb geochronologic data provide precise constraints on the age of several key stratigraphic levels in the Neoproterozoic Huqf Supergroup and its subjacent crystalline basement rocks. The basement ages constrain an interval of felsic magmatism to have occurred from at least 840 Ma to approximately 810 Ma. Detrital zircons from several stratigraphic levels within the Huqf Supergroup yield ages in excess of 2.5 Ga, suggesting proximity of Archean crust during the Neoproterozoic evolution of the eastern Arabian Peninsula. Volcanic ash beds intercalated within the Huqf Supergroup were dated in the Oman Mountains, and in several subsurface wells (South Oman Salt Basin). Glacial deposits of the Abu Mahara Group in the Oman Mountains (Ghubrah Formation) contain volcaniclastic rocks that are approximately 713 Ma; overlying syn-glacial turbiditic sandstones of the Fiq Formation yield a suite of detrital zircon dates ranging from 920 to 664 Ma so that deposition of at least the upper Fiq must have post-dated 664 Ma. In the South Oman Salt Basin, volcaniclastic deposits intercalated within glaciogenic strata of the Fiq Formation yielded zircons, the youngest of which is about 645 Ma. These data indicate two distinct episodes of glaciation at approximately 713 and <645 Ma.

The uppermost Ara Group of the Huqf Supergoup contains multiple ash beds within its carbonate strata, where an age of roughly 547 Ma is reported for rocks that occur above strata marked by a pronounced negative (-12‰) to positive (+4‰) excursion in carbon isotope composition. Higher in the Ara sequence, three distinct ash beds dated at about 543 Ma, 542 Ma, and 541 Ma closely approximate the Ediacaran-Cambrian boundary in Oman. The dramatic carbon isotope excursion of ∼16 permil in the Shuram Formation (middle Nafun Group) has a firm maximum age of approximately 620 Ma as provided by detrital zircon ages from the base of the formation. Interpolation and downward extrapolation from the Ara Group ages, coupled with correlation to other global strata, suggests the base of the Shuram C-isotope excursion to be on the order of 560 Ma, with an estimated duration of approximately 5 to 11 m.y. This excursion is inferred to post-date the last well-documented Neoproterozoic glaciation (about 582 Ma) and is broadly coincident with the appearance of complex organisms in the fossil record.