Eurypterids
Specimen of the Month
Samuel J. Ciurca, Jr., Rochester, New York        by  Eurypterids.net
JANUARY 2010
The Great Late Silurian Earthquake
Scajaquada Seismite 2003
MORE EURYPTERID OF THE MONTH PAGES  
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ABOVE: A Late Silurian stratigraphic section, documented for the first time, showing part of the eurypterid-bearing
sequence of the Bertie Group of Ontario, Canada. The massive bed (hammer) shown is the Williamsville 'A' member of
the Williamsville Formation. Below are the uppermost beds of the Scajaquada Formation that are unfossiliferous, but
do contain evidence of salt (halite) deposition and at
'S' what I believe is a seismite, i.e. a bed resulting from much
disturbance due to a Late Silurian earthquake that 'rattled' the lime muds, forming a brecciated bed. The above photo
was taken in 2003 on a visit to a quarry to collect eurypterid remains from the Williamsville Formation. Argillaceous
material within the unit causes relatively rapid decomposition of the beds, but the more resistant parts, including thin
waterlimes, stand out in relief.
In stratigraphically lower beds, i.e. the Ellicott Creek Breccia (Fiddlers Green Formation), an inferred seismite was
deemed responsible for the widespread occurrence of the brecciated layers within that unit (Ciurca,
S
Williamsville 'A' Waterlime
Scajaquada Formation
LEFT: A block of Scajaquada Formation from
Fort Erie, Ontario, Canada (RQS) showing very
irregular (disturbed) bedding and stringers of
sandy material and pyrite. This block was re-
trieved on November 20, 2009 from uppermost
beds of the formation. Small chert nodules are
often observed and have been traced eastward
to at least Victor, New York (Mud Creek).
To the east, in the Finger Lakes Region of New
York, this unit grades into evaporites
(gypsum/anhydrite) of the Forge Hollow Fm.
The basin center is presumed to be within this
region.
Counter
LEFT: A slice of Ellicott Creek
Breccia from the Neid Road Quarry
north of LeRoy, New York. The
section may be part of a stroma-
tolite that was mutilated by a
paleoearthquake. The slab is not
polished, but was photographed
under water. This specimen was
collected from the upper beds of the
Ellicott Creek Formation, perhaps
what I have called 'B' at the type
section at Williamsville, New York.
RIGHT: The side of the block of Scajaquada
Formation (above) shows a nodule of Chert
(about 1.0 cm) slowly weathering out with
smaller pieces of the mineral to the right.
A very thin layer of waterlime in the lowest beds
of the Scajaquada Fm. has yielded fine
eurypterid remains associated with salt
hoppers indicating that the entire unit is part of
a Late Silurian evaporite basin, a continuation
of the thick Salina Group evaporite sequence
so widespread in the Appalachian Basin.
Bands of WATERLIME
DOLOMITIC MUDSTONE
North
South
ABOVE: The author collecting a chunk of the Ellicott Creek Breccia at the NY-88 roadcut at Phelps, New York. This eurypterid-bearing
waterlime is chaotically brecciated and is likely a continuation of the paleoseismite observed westward to Buffalo and into the Niagara
Peninsula of Ontario, Canada.
CLICK HERE to see the sample of the breccia removed (November 28, 2009). The heavy beds above are of
Devonian age as the Silurian-Devonian Boundary occurs here.
BLUE
Breccia Bed
Note added December 25, 2009
Just received a book I ordered and offer as
suggested reading:
"Ancient Seismites"
Editors: F. F. Ettensohn, N. Rast and C. E. Brett
Special Paper 359, Geological Society of
America, 2002.