| Desiccation - Preservation of Mudcracks
Samuel J. Ciurca, Jr., Rochester, N.Y.
Hughmilleria socialis - pair of individuals collected November 30, 2006 from the Pittsford Member of the Vernon Fm. (Salina
Group), Tastings Site, Pittsford, New York. Coloration is due to iron mineralization on eurypterid integument undoubtedly due to
infiltration of groundwater along bedding planes. The specimen (CIURCA xxxxxx-1) on the right, about 14 cm in length, is a venter
showing the underside of this common eurypterid. The specimen to the left is mostly black (carbonized) and a portion of the anterior is
covered by matrix. Since cracks developed after the specimen dried out, these had to be reinforced with glue. A little more weathering of
this delicate shale would result in a pile of crumbled bits.
ABOVE: July 20, 2006, piles of relatively fresh rock - a mixture of the various subunits of the Pittsford Member and overlying redbeds and
waterlime and underlying Blue Bed (see stratigraphic chart for the Spring House Commons Site). The first rain will start to totally destroy
most of this rock. Fossils need to be removed as soon as possible. The resistant, intercalated waterlimes, sometimes help to support
overlying and underlying shale (and any fossils therein) and protect some layers from rapid decomposition (and any fossils therein).
Many of the other photos on this site show the excavation, collecting, and decomposition of the piles shown above.
|Sedimentary Structures, Tastings Site, Pittsford, New York
LEFT: MUDCRACKS - A beautiful example from a
slab exhibiting weathering that accentuates the
more resistant layers within the sediment. These
structures, in one form or another, are abundant
throughout the Pittsford Member and in overlying
beds of green and red shale.
Such desiccation cracks indicate deposition in very
shallow waters - extensive mudflats probably with
shallow lagoons and rapidly changing flows of
mud and calcareous (dolomitic) intercalations.
As seen below, eurypterid remains were often
deposited on the mudflats and then subjected to
desiccation. The example show here is a carapace
of Hughmilleria socialis - note the dislocation of the
carapace by the mudcrack.
LEFT: Halite crystal impressions, preserved as salt
hoppers within the Slate Bed (Pittsford B). This type
of structure is commonly encountered within many
eurypterid-bearing units within the Salina and Bertie
Groups of New York. Salt hoppers up to one foot on
a side have been observed within the Bertie Group.
ABOVE & LEFT: A slab of 'slaty' shale preserving mudcracks and
collected from the Tastings Site during the Autumn of 2006. Actual
color of the rock is a beautiful blue in sunlight. The horizon from which
the slab originated is probably just above the Pittsford Member, but
illustrates the common occurrence of these structures throughout the
lower Vernon Fm. sequence even into the overlying redbeds. 12/07/06
RIGHT: A small slab, about 14 cm in length, with abundant
ostracods and a few eurypterid fragments preserved. The
fragment at the far right is part of the telson of Hughmilleria
socialis. Within the Pittsford Member, ostracods are often
encountered in clusters on bedding planes and often in
the more dolomitic mudstones. Also as abundant 'ghosts'
within the Slate Bed and commonly superimposed on
eurypterid integument. CLICK PHOTO TO ENLARGE