Genesis and geometry of the Meiklejohn Peak lime mud-mound, Bare Mountain Quadrangle, Nevada, USA: Ordovician limestone with submarine frost heave structures—a possible response to gas clathrate hydrate evolution

Abstract

Abstract During the Early Middle Ordovician (Early Whiterockian) the Meiklejohn Peak lime mud-mound, a large whaleback or dolphin back dome, grew on a carbonate ramp tens to hundreds of kilometres offshore. This ramp extended from the northwest margin of Laurentia into the open waters of the ancestral Pacific Ocean to the north. The mound developed in an outer ramp environment, in relatively deep and cold water. A steep northern margin with a slope that exceeds 55° characterizes the mound. This margin is split by a 14-m long vertical fracture that separates a zone of slumped, drag-folded and brecciated rocks from the main mass of the mound. Failure along this fracture occurred subcutaneously, as highlighted by covering beds that are folded next to the mound. Brecciated blocks and clasts contain zebra and stromatactis structures indicating that these rocks and structures were lithified early in the history of the mound. The southern end of the mound is less steep and is characterized by large, echinodermal grainstone cross-beds. These deposits are part of a large, subaqueous dune that grew northwards and preceded the main development of the mound. Southward dipping and downlapping layers of mud-mound mudstone and wackestone overlie the dune. These muddy limestone layers are cut in several places by injection dykes and are pierced, near the contact with the underlying dune, by a 25-m long pipe filled with rotated nodular and brecciated mud-mound clasts. This long pipe extends to the edge of the mound and appears to have been a conduit where fluidized materials that came from the mound's interior were vented. The interior of the mound is typified by light grey limestone with zebra bands and stromatactis structures. Both structures represent former cavity systems that are filled with fibrous and bladed calcite and pelleted and laminated geopetal mudstone. Spar bands of zebra limestone often extend for several metres and appear to have been unsupported over these distances. Zebra banded rocks are also accompanied by snout and socket structures and, in some instances, are folded and sheared by curving kink bands. Zebra and stromatactis limestone structures found throughout the mud-mound resemble frost heave and cryoturbation structures identified in both Holocene and Pleistocene cryosols, and in laboratory experiments with advancing freezing fronts in clay-size sediment. Significantly, modern occurrences of methane clathrate hydrate (methane-charged ice) display parallel and digitate layering similar in depositional appearance to that of zebra and stromatactis limestone from Meiklejohn Peak. Early carbonate cements are also commonly associated with these modern clathrate hydrate deposits. Consequently, gas clathrate hydrates may have been the propping agent for zebra and stromatactis structures observed in the mud-mound. In this scenario, carbonate cements would have precipitated and stabilized these structures, both with the consolidation and dissociation of gas clathrate hydrates, and with the oxidation and reduction of associated gases. Stable δ 13 C and δ 18 O isotope ratios collected from mudstone and spar of zebra and stromatactis structures indicate that they were lithified in equilibrium with Ordovician seawater. The δ 13 C isotope ratios recorded at Meiklejohn Peak are similar to δ 13 C isotopic ratios obtained from ∑CO 2 evolving from modern seafloor. These isotopic ratios may indicate that frost heave structures in the Meiklejohn Peak mud-mound are the result of consolidation and dissociation of carbon dioxide clathrate hydrates. Even though the bulk of gas clathrate hydrates identified to date in modern ocean floors are composed of methane, carbon dioxide clathrate hydrates are known from the modern seafloor of the Okinawa Trough. They may also be common in areas of abundant carbonate sediment accumulation, as suggested by recent observations from the Great Australian Bight.