Mazon Creek - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 123

127 Rapid sedimentation is characteristic of

conditions adjacent to a delta. Cores
resulting from boreholes drilled to test the
foundation of the nuclear power plant in the
vicinity of Pit 11 revealed complete
sedimentation records. Moreover, the
clay-silt laminae in these cores are paired,
and the pairs widen and narrow in a cyclical
fashion ( 127 ). Kuecher et al . (1990) studied
the cores and the cyclicity of the paired
laminae and interpreted the cyclicity as tidal
in origin. The thin clay bands represent still-
stands, either flood slack or ebb slack, i.e.
when the tide is in the process of turning
and the water is not flowing in either
direction ( 128 ). The thicker silt layers
represent periods of greater deposition
(from the landward direction). The thicker
silt bands were laid down during the ebb
tide, when the outgoing tide allowed a high
water flow into the basin; the narrower silt
bands represent flood tides, when the
incoming tide resists the flow to some
extent. Thus, a single tidal cycle consists of
two clay band and two silt bands. The widest
bands correspond to spring tides, with the
highest tidal range, and the narrowest to
neaps.
Kuecher et al . (1990) found that there
were 15-16 tides in a cycle from widest to
next widest. This corresponds to half a
lunar month; a complete lunar cycle has
two springs (when the Moon and Sun align
with the Earth) and two neaps (when the
Moon-Earth axis is perpendicular to the
Sun-Earth axis). Evidence from cyclicity of
coral growth (Johnson and Nudds, 1975)
suggests that the lunar month consisted
of 30 days in the Pennsylvanian period
(i.e. the Earth-Moon system has quickened
over 250 million years to today's
28-day lunar cycle). Thus, the tidal cycle at
Mazon Creek was the diurnal type, as found
today in some parts of the world such as the
Gulf of Mexico; tides around British coasts
are semi-diurnal (i.e. there are two highs
and two lows in a 24-hour period).
The tidal cyclicity provides a rare, direct
measure of sedimentation rate. Each
fortnightly cycle measures from 19-85 mm
(0.75-3.5 in) in thickness. This provides

127 Cycles of clay silt pairs in the Francis
Creek Shale laminites. MU. Scale bar is in mm.

a deposition rate of about 0.5-2.0 m
(1.6-6.6 ft) per year of compacted
sediment. The entire Francis Creek
Shale was therefore deposited in 10-50
years. The tidal cycles provide
independent, quantitative evidence of
rapid sedimentation already concluded
from qualitative evidence from sediments
and fossils.

C OMPARISON OF THE M AZON
C REEK WITH OTHER U PPER
P ALEOZOIC BIOTAS
Calver (1968) recognized a sequence of
onshore-offshore communities in the
hard-part fossil record of the Westphalian
strata of northern England. His estheriid
association roughly corresponds to the