Allochthonous slope sequence at Lobster Cove Head: evidence for a complex Middle Ordovician platform margin in western Newfoundland

Abstract

The upper part of an intact sequence of Lower to Middle Ordovician deep-water sediments, which now form a large, disrupted raft within the Rocky Harbour Mélange at Lobster Cove Head, is interpreted as having been deposited downslope from a drowned carbonate platform margin. The entire 50 m thick section is Arenig (late Canadian or Ibexian to early Whiterock) in age; graptolite biostratigraphy demonstrates a correlation with upper parts of the Cow Head Group to the north. The basal part of the section is a proximal facies of the Cow Head Group (Shallow Bay Formation, Factory Cove Member, Beds 9, 10, and part of Bed 11). The upper part of the section consists of interbedded dolostone and shale and is unlike any other sequence in the Cow Head Group. This upper sedimentary sequence is defined as the Lobster Cove Head Member of the Shallow Bay Formation, Cow Head Group. Contact between the two sedimentary packages is also marked by a faunal break and coincides with emplacement of megaconglomerate Bed 12 at Cow Head.This break marks the change from a uniform to complex carbonate platform margin configuration and is here interpreted as the result of synsedimentary faulting. The margin upslope from Cow Head remained in shallow water during the final stages of Cow Head Group deposition, whereas that upslope from Lobster Cove Head was drowned and shed little sediment into deep water. The synsedimentary faulting, which led to rapid subsidence and platform-margin drowning upslope from Lobster Cove Head and possibly the deposition of megaconglomerate Bed 12 at Cow Head, coincides with the onset of the Taconic Orogeny in western Newfoundland.

Active synsedimentary tectonism on a mixed carbonate-siliciclastic continental margin: third-order sequence stratigraphy of a ramp to basin transition, lower Sekwi Formation, Selwyn Basin, Northwest Territories, Canada

“…The Lower Cambrian Shady Dolomite of the central Appalachians developed a steep rimmed shelf margin that had 700 to 1000 m of relief and passed basinward into peri‐platform talus consisting primarily of shallow‐water facies interbedded with deep basinal mudstones (Barnaby & Read, 1990). The Cambro‐Ordovician Cow Head Group of western Newfoundland also developed rimmed shelf margins that passed basinward into thick successions of downslope conglomerates (Hiscott & James, 1985; Coniglio, 1986; James & Stevens, 1986; James et al. , 1987).…”

Middle Ordovician (middle Darriwilian) Archaeospicularia and Entactinaria (radiolarians) from the Table Cove Formation, Piccadilly Quarry, Newfoundland, Canada

“…( 1 ) Lithostratigraphic log (after Maletz and Egenhoff, 2011; Kachovich and Aitchison, 2020) showing the sampled levels (PD01, -03, -05, -07, -09, -11, -12, and -13). Generalized stratigraphy after Knight (1991) and James et al (1987); ( 2 ) field photo, facing southwest, of the exposure of gently dipping beds of rhythmically bedded limestone. Reprinted with permission from Kachovich and Aitchison (2020, fig.…”

Micro-CT study of Middle Ordovician Spumellaria (radiolarians) from western Newfoundland, Canada

“…Stratigraphic context of the Table Cove Formation at the Piccadilly Quarry: (1) lithostratigraphic log after Maletz and Egenhoff (2011) showing the sampled levels (PD01, -03, -05, -07, -09, -11, -12, and -13). Generalized stratigraphy after Knight (1991) and James et al (1987); (2) field photo, facing southwest, of the exposure of gently dipping beds of rhythmically bedded limestone. Remarks.-Polycystines with radial arrangement of numerous spines with a distinct internal microsphere and without an internal spicular system.…”