Cyclicity and Community Replacement in the Rochester Shale
Large-Scale Cycle: Overall, the Irondequoit Limestone, Rochester Shale and DeCew Dolostone form a large-scale deepening-shallowing cycle on the scale of perhaps a million years. It is an unconformity bounded package or depositonal sequence, identified by Brett et al. (1990, 1998) as Silurian sequence V. This sequence is bounded below by a sharp unconformable contact of the basal crinoidal limestones of the Irondequoit and at the top by a distinctly erosional contact at the base of the crinoid-rich and reefy Gasport Limestone of the Lockport Group. This unconformity progressively and rapidly cuts out the DeCew Formation and the thin Rochester Shale northwest of Hamilton, Ontario.
Like most depositional sequences, Sequence V is not symmetrical probably because the types and rates of sedimentation vary predictably during a rise and fall of base level. During transgressions, the effect of raising base level is to starve offshore areas of mud and silt, which is trapped or sequestered in shallow bays and estuaries leading to clean platforms offshore. The net effect of this offshore starvation of mud and silt is to allow buildup of skeletal limestones, such as the Irondequoit Limestone. Conversely, during sea level stillstand and fall, seaward regression of the shoreline occurs allowing greater amounts of increasingly coarse clastic sediments to be deposited offshore, as recorded in the upper Rochester (Burleigh Hill Member) and DeCew formations.
The Irondequoit-Rochester succession has been traced widely in eastern North America and may have counterparts in Europe and elsewhere. As noted above, a similar succession of limestone into shale is recorded in the Welsh Borderland of Great Britain, which during the Silurian, was on a separate microcontinent termed Avalonia, and a very similar succession exists in strata of the same age in the Swedish island of Gotland and in northwestern Estonia (Brett, unpublished), which were parts of the paleocontinent of Baltica or Ancestral Europe (Cocks, 2001). The occurrence of similar transgressive-regressive successions on at least three separate paleocontinents strongly indicates that this fluctuation was produced by a global or eustatic cycle of deepening and shallowing on the scale of tens of meters of water. And even smaller scales of cycles appear to be correlatable. If so, this provides possible evidence for glacial retreat and buildup in the middle Silurian, as this is the primary mechanism responsible for global rise and fall of sea level. Ancient glacial tillite deposits are known from the early Silurian strata of the Paleoandes in South America, which were then closer to the South Pole (Caputo et al., 1998). One of the later glacial deposits may correspond with the unconformity below the Irondequoit.
Smaller-Scale Cycles: Nested within the Irondequoit-Rochester-DeCew large-scale cycle (10 - 40 meters) are two smaller cycles (Irondequoit to lower Lewiston and upper Lewiston E-submember to DeCew sequences), and a series of even smaller meter-scale cycles. The Lewiston Member was long recognized to record a nearly symmetrical cycle of sedimentation probably associated with water depth change of a few tens of meters
(Brett 1982, 1983), and reflected in cyclic changes in biofacies (Tetreault 1994). New data from the Middleport area provide strong support for this hypothesis and add important details (Figures 2 and 5).
The lower Rochester cycle starts with a strong deepening recorded in the upper Irondequoit Limestone into the lower Lewiston Member. The sharp contact between the Irondequoit and the basal Rochester Shale is interpreted as a flooding surface, associated with rather rapid rise of sea level, which produced the effect of sediment starvation offshore. Commencing with shallow shelf conditions near normal wave base (~5 - 10 meters of water) water deepened into deeper subtidal conditions occasionally scoured by storms but not effected by fair weather waves. The occurrence of mounds of fine, micritic limestone and bryozoans at the surface attests to a period of upward growth by organisms during a time of clear water and probably base level rise. Certain organisms were closely related to the Irondequoit mounds.
Large-Scale Cycle: Overall, the Irondequoit Limestone, Rochester Shale and DeCew Dolostone form a large-scale deepening-shallowing cycle on the scale of perhaps a million years. It is an unconformity bounded package or depositonal sequence, identified by Brett et al. (1990, 1998) as Silurian sequence V. This sequence is bounded below by a sharp unconformable contact of the basal crinoidal limestones of the Irondequoit and at the top by a distinctly erosional contact at the base of the crinoid-rich and reefy Gasport Limestone of the Lockport Group. This unconformity progressively and rapidly cuts out the DeCew Formation and the thin Rochester Shale northwest of Hamilton, Ontario.
Like most depositional sequences, Sequence V is not symmetrical probably because the types and rates of sedimentation vary predictably during a rise and fall of base level. During transgressions, the effect of raising base level is to starve offshore areas of mud and silt, which is trapped or sequestered in shallow bays and estuaries leading to clean platforms offshore. The net effect of this offshore starvation of mud and silt is to allow buildup of skeletal limestones, such as the Irondequoit Limestone. Conversely, during sea level stillstand and fall, seaward regression of the shoreline occurs allowing greater amounts of increasingly coarse clastic sediments to be deposited offshore, as recorded in the upper Rochester (Burleigh Hill Member) and DeCew formations.
The Irondequoit-Rochester succession has been traced widely in eastern North America and may have counterparts in Europe and elsewhere. As noted above, a similar succession of limestone into shale is recorded in the Welsh Borderland of Great Britain, which during the Silurian, was on a separate microcontinent termed Avalonia, and a very similar succession exists in strata of the same age in the Swedish island of Gotland and in northwestern Estonia (Brett, unpublished), which were parts of the paleocontinent of Baltica or Ancestral Europe (Cocks, 2001). The occurrence of similar transgressive-regressive successions on at least three separate paleocontinents strongly indicates that this fluctuation was produced by a global or eustatic cycle of deepening and shallowing on the scale of tens of meters of water. And even smaller scales of cycles appear to be correlatable. If so, this provides possible evidence for glacial retreat and buildup in the middle Silurian, as this is the primary mechanism responsible for global rise and fall of sea level. Ancient glacial tillite deposits are known from the early Silurian strata of the Paleoandes in South America, which were then closer to the South Pole (Caputo et al., 1998). One of the later glacial deposits may correspond with the unconformity below the Irondequoit.
Smaller-Scale Cycles: Nested within the Irondequoit-Rochester-DeCew large-scale cycle (10 - 40 meters) are two smaller cycles (Irondequoit to lower Lewiston and upper Lewiston E-submember to DeCew sequences), and a series of even smaller meter-scale cycles. The Lewiston Member was long recognized to record a nearly symmetrical cycle of sedimentation probably associated with water depth change of a few tens of meters
(Brett 1982, 1983), and reflected in cyclic changes in biofacies (Tetreault 1994). New data from the Middleport area provide strong support for this hypothesis and add important details (Figures 2 and 5).
The lower Rochester cycle starts with a strong deepening recorded in the upper Irondequoit Limestone into the lower Lewiston Member. The sharp contact between the Irondequoit and the basal Rochester Shale is interpreted as a flooding surface, associated with rather rapid rise of sea level, which produced the effect of sediment starvation offshore. Commencing with shallow shelf conditions near normal wave base (~5 - 10 meters of water) water deepened into deeper subtidal conditions occasionally scoured by storms but not effected by fair weather waves. The occurrence of mounds of fine, micritic limestone and bryozoans at the surface attests to a period of upward growth by organisms during a time of clear water and probably base level rise. Certain organisms were closely related to the Irondequoit mounds.
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