Bertie Formation


The Bertie Formation, also called Bertie Limestone and Bertie Dolomite, formerly named as Bertie Group, is a Late Silurian geologic formation and Lagerstätte in southern Ontario, Canada and western New York State, United States. The formation comprises dolomites, limestones and shales and reaches a thickness of in the subsurface, while in outcrop the formation can be thick.
The formation represents the uppermost unit of the Salina Group and the youngest Silurian formation in Ontario. The formation overlies the Camillus Formation and is in Ontario conformably overlain by the Bois Blanc Formation and in New York by the Onondaga Limestone, both dating to the Devonian.
The Bertie Formation is considered a Konservat-Lagerstätte; a formation renowned for the preservation of a unique fauna. The formation has provided many fossils of Silurian eurypterids and Proscorpius osborni, the oldest known scorpion, as well as primitive fossil flora and the fish Nerepisacanthus denisoni. The excellent preservation of the many eurypterids possibly was the result of periodic hypersaline and anoxic conditions.

Description

The type locality for the Bertie Formation is in the Ridgemount Quarry, located west of Fort Erie on the Niagara Peninsula of Bertie, Ontario, west of Buffalo, New York, after which the formation is named. The first author who recognized the formation as a separate stratigraphic unit was Chapman in 1884. In older literature and other authors maintain this definition, the formation was named Bertie Group.

Geographic extent

The Bertie Formation forms the bedrock in a narrow band extending from Fort Erie, west of Buffalo, New York, through Hagersville, New Hamburg, Harriston, and Walkerton to Southampton on Lake Huron. The formation consists of medium- to massive-bedded aphanitic brown to grey, laminated, bituminous and burrowed dolomites, with minor thin-bedded shaly dolomites.
Along the outcrop area between Fort Erie and Hagersville, the thickness varies from. It thickens to in the subsurface. Sanford used the term Bertie Formation from Fort Erie to the vicinity of Hagersville and the term Bass Islands Formation north and west of Hagersville. The formation is correlated with the Bass Islands Formation of Michigan. The Bertie Dolomite is quarried for crushed stone at Fort Erie, Port Colborne, Dunnville, Cayuga, and Hagersville. It is the uppermost formation of the Silurian System in Ontario, dating to the Pridoli epoch, or in the local chronology Cayugan or Murderian.

Stratigraphy

The Bertie Formation is the uppermost unit of the Salina Group and forms part of the Tippecanoe II sequence. At its type locality, the formation is subdivided into the Falkirk Dolomite, Scajaquada Shale, and Williamsville Dolomite members. In central New York, the formation is subdivided into the Fiddlers Green Dolomite, Forge Hollow Shale, and Oxbow Dolomite members, from oldest to youngest. Here, the Bertie Formation is overlain by the Cobleskill Dolomite, also named Akron Member. In New York, the Onondaga Limestone overlies the Bertie Formation. The formation is in Ontario conformably overlain by the Middle Devonian Bois Blanc Formation.
Laterally, the formation is equivalent to the Bass Islands Formation and is mapped as a combined stratigraphic unit. Haynes and Parkins reported that the
Bertie Formation is progressively cut by the Bass Islands Formation from Dunnville to Hagersville. In Pennsylvania, the formation is time-equivalent with the Keyser Formation.

Fossils

The Bertie Formation is considered a Konservat-Lagerstätte; a formation characterized by a rich and well-conserved fauna. The formation has provided more than 800 fossil eurypterids. Of just Eurypterus lacustris almost 600 specimens were found in the Williamsville Member. Other fossils from the formation are Proscorpius osborni, the oldest known scorpion, flora and a fossil fish; Nerepisacanthus denisoni. The excellent preservation of the many eurypterids possibly was the result of periodic hypersaline and anoxic conditions.
GroupFossilsMemberImageNotes
FishNerepisacanthus denisoniWilliamsville
EurypteridsAcutiramus macrophthalmusFiddlers Green
Williamsville
EurypteridsBuffalopterus pustulosusWilliamsville
EurypteridsCarcinosoma scorpionisOxbow Dolomite
EurypteridsClarkeipterus testudineusFiddlers Green
EurypteridsDolichopterus herkimerensisFiddlers Green
EurypteridsD. jewettiFiddlers Green
EurypteridsD. macrocheirusWilliamsville
EurypteridsD. siluricepsFiddlers Green
Williamsville
EurypteridsErieopterus pustulosusFiddlers Green
EurypteridsEurypterus dekayiWilliamsville
EurypteridsE. laculatusFiddlers Green
EurypteridsE. lacustrisWilliamsville
EurypteridsE. remipesWilliamsville
EurypteridsEusarcana scorpionisWilliamsville
EurypteridsPterygotus cobbiWilliamsville
EurypteridsP. cummingsiFiddlers Green
EurypteridsP. grandisFiddlers Green
EurypteridsP. juvensisFiddlers Green
EurypteridsP. macrophthalmusFiddlers Green
Williamsville
EurypteridsErettopterus sp.Fiddlers Green
EurypteridsRhinocarcinosoma sp.Williamsville
PlanatergaBunaia woodwardiFiddlers Green
PlanatergaLimuloides eriensisFiddlers Green
PlanatergaPseudoniscus clarkei, P. rooseveltiFiddlers Green
ArachnidsPalaeophonus osborniFiddlers Green
ArachnidsProscorpius osborniFiddlers Green
MachaeridiansLepidocoleus reinhardiFiddlers Green
GraptolitesClimacograptus ultimusFiddlers Green
GraptolitesPalaeodictyota buffaloensisFiddlers Green
GraptolitesAscograptus sp.Fiddlers Green
GraptolitesOrthograptus sp.Fiddlers Green
CrustaceansCeratiocaris aculeatus, C. maccoyanaFiddlers Green
CrustaceansEmmelezoe minutaFiddlers Green
CrustaceansGonatocaris sp.Williamsville
CephalopodsDawsonoceras oconnellaeFiddlers Green
CephalopodsGomphoceras ruedemanniFiddlers Green
CephalopodsMitroceras gebhardiFiddlers Green
CephalopodsPristeroceras timidumFiddlers Green
CephalopodsTrochoceras cf. anderdonenseFiddlers Green
CephalopodsOrthoceras sp.Fiddlers Green
CephalopodsPhragmoceras sp.Fiddlers Green
GastropodsLoxonema bertienseFiddlers Green
GastropodsMesocoelia gregariaFiddlers Green
GastropodsPlatyceras sp.Fiddlers Green
Bivalves"Hercynella" buffaloensisFiddlers Green
BivalvesModiolopsis dubiusFiddlers Green
BivalvesGoniophora sp.Fiddlers Green
BivalvesNuculites sp.Fiddlers Green
BivalvesRhytimya sp.Fiddlers Green
OstracodsEukloedenella umbilicataFiddlers Green
OstracodsLeperditia alta, L. scalarisFiddlers Green
OstracodsZygobeyrichia cf. reginaFiddlers Green
PolychaetaSerpulites sp.Fiddlers Green
PolychaetaSpirorbis sp.Fiddlers Green
RhynchonellataCamarotoechia cf. andrewsiFiddlers Green
RhynchonellataReticularia modestoideaFiddlers Green
RhynchonellataHowellella eriensisFiddlers Green
RhynchonellataWhitfieldella sulcataFiddlers Green
RhynchonellataDelthyris sp.Fiddlers Green
EdrioasteroideaPyrgocystis batheriFiddlers Green
StrophomenataSchuchertella sp.Fiddlers Green
BryozoansHernodia sp.Fiddlers Green
BryozoansReptaria sp.Fiddlers Green
BryozoansStigmatella sp.Fiddlers Green
LingulataLingula semina, L. subtrigonaFiddlers Green
LingulataOrbiculoidea cf. numulusFiddlers Green
ScyphozoaMetaconularia perglabraFiddlers Green
AnthozoaAulocystis sp.Fiddlers Green
Anthozoa?Ceratopora sp.Fiddlers Green
StromatoporoideaStromatopora constellataFiddlers Green
DasycladophyceaeMedusaegraptus graminiformisFiddlers Green
FloraInocaulis lesqueureuxiWilliamsville
FloraCooksonia sp.Williamsville
FloraHostinella siluricaFiddlers Green
CyanobacteriaMorania bertiensisFiddlers Green
CyanobacteriaCalithamnopsis siluricaFiddlers Green

Age

The Bertie Formation is Late Silurian.

Interpretations of depositional environments

The Appalachian Foreland Basin was formed during the Alleghanian orogeny in the Early to Middle Ordovician. The period of mountain building led to the closure of the Iapetus and Rheic Oceans. Due to tectonic loading, the foreland basin developed in the present-day area north of the Appalachian Mountains. The Late Silurian was represented by the deposition of evaporites, underlying the Bertie Formation in the Salina Group, ranging in thickness from. The evaporites and related dolomites and limestones were deposited in a warm, arid climate shallow water restricted basin. Zones of stromatolites and thrombolites occur at several horizons in the Fiddlers Green member of the formation, as well as desiccation cracks exist in the Bertie Formation. During the Hercynian orogeny in the Devonian, many of the Silurian sediments were eroded to the south in the Appalachians, while north of the mountains the Silurian units were preserved.
The sediments of the Bertie Formation were deposited on the paleosouthern side of the subsiding Algonquin Arch, flanking the northern rim of the Appalachian foreland basin of Laurentia.
The Bertie Formation was deposited in a hypersaline marine environment. The stratigraphic sections and the fossil content suggest that the formation was deposited in a near-shore marine to lagoonal setting, and the evaporites and casts of halite pseudomorphs with sides of up to, suggest the environment was far from normal marine; hypersalinity must have prevailed throughout most of the depositional history of the formation. Alternating hypersaline and brackish estuarine conditions have been recorded in the formation. The dolomitization of the formation most probably was not primary.