The sedimentology of the lower Silurian whirlpool sandstone in subsurface Lake Erie, Ontario

摘要

The lower Silurian Whirlpool Sandstone is composed of two main units: a fluvial unit andan estuarine to transitional marine unit. The lowermost unit is made up of sandy braidedfluvial deposits, in shallow valleys, that flowed towards the northwest. The fluvial channelsare largely filled by cross-bedded, well sorted, quartzose sands, with little ripple crosslaminatedor overbank shales.Erosionally overlying this lower unit are brackish water to marine deposits. In the east,this unit consists of estuarine channels and tidal flat deposits. The channels consist of fluvialsands at the base, changing upwards into brackish and tidally influenced channelizedsandstones and shales. The estuarine channels flowed to the southwest. Westwards, the unitcontains backbarrier facies with extensive washover deposits. Separating the backbarrierfacies from shoreface sandstone facies to the west, are barrier island sands represented bybarrier-foreshore facies. The barrier islands are dissected by tidal inlets characterized byfining upward abandonment sequences. Inlet deposits are also present west of the barrierisland, abandoned by transgression on the shoreface. The sandy marine deposits are replacedto the west by carbonates of the Manitoulin Limestone.During the latest Ordovician, a hiatus in crustal loading during the Taconic Orogeny led toerosional offloading and crustal rebound, the eroded material distributed towards the west,northwest and north as the terrestrial deposits of the fluvial Whirlpool. The "anti-peripheralbulge" of the rebound interfered with the peripheral bulge of the Michigan Basin, nulling theAlgonquin Arch, and allowing the detritus of the fluvial Whirlpool to spread onto theAlgonquin Arch.The Taconic Orogeny resumed in the earliest Silurian with crustal loading to the south andsoutheast, and causing tilting of the surface slope in subsurface Lake Erie towards theiisouthwest. Lowstand terrestrial deposits were scoured into the new slope. The new crustalloading also reactivated the peripheral bulge of the Appalachian Basin, allowing it to interactwith the bulge of the Michigan Basin, raising the Algonquin Arch. The crustal loadingdepressed the Appalachian basin and allowed transgression to occur. The renewedAlgonquin Arch allowed the early Silurian transgression to proceed up two slopes, one to theeast and one to the west. The transgression to the east entered the lowstand valleys andcreated the estuarine Whirlpool. The rising arch caused progradation of the Manitoulincarbonates upon shoreface facies of the Whirlpool Sandstone and upon offshore facies of theCabot Head Formation. Further crustal loading caused basin subsidence and rapidtransgression, abandoning the Whirlpool estuary in an offshore setting.