Rocky Shorelines - Trace Fossils as Indicators of Sedimentary Environments - page 439

sands disconformably overlie the Upper Silurian dolomitic Bertie Formation.
A second disconformity exists between the two Devonian sandy units. The
top surface of the Bertie Formation has a local undulatory topography of up
to 1 m with the presence of some dolomitic blocks. The dolomitic substrate
bears a system of joints and fractures, which were widened by dissolution where
Springvale erosion removed the Oriskany sands. Other evidences of exposure
and karstification include vug and cavern porosity. Trypanites borings penetrate
the Silurian dolomites in the top horizontal discontinuity and in the walls of
open fractures of both, the pre- and the post-Oriskany disconformity. They
are tubular, rectilinear to gently curved, 2-3 mm in diameter and 10-30 mm
long. This morphology is comparable to the borings of some modern sipunculid
worms ( Pemberton et al., 1980 ). Algal microborings originate from the walls of
Trypanites . These bioerosional features were formed during transgression fol-
lowing substrate emersion and karstification. No other evidence of marine sub-
mersion, such as epilithozoan skeletons, was found.
5.2 Middle Jurassic, England
In the Mendip Hills, west of Frome (Somerset), Carboniferous (Dinantian) lime-
stones are overlain by Jurassic units ( Wall and Jenkyns, 2004 ). One of the first
researchers studying this unconformity was De La Beche (1846) , who recog-
nized borings and provided some of the first illustrations of fossil bioerosion
on rocky shores ( Fig. 6 A and B). Bromley (1975 : fig. 18.6) pointed out the
FIGURE 6 Jurassic rocky-shore trace fossils from England. (A, B) Original figures by De La
Beche (1846) showing Trypanites and Gastrochaenolites . (C) Section of a Carboniferous limestone
with Jurassic Trypanites and Gastrochaenolites .