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1991. M.Sc., 372 pp.
THE
FLOODPLAIN DEPOSITS AND PALEOSOL PROFILES OF THE LATE CARBONIFEROUS CUMBERLAND
COAL BASIN, EXPOSED AT JOGGINS, NOVA SCOTIA, CANADA
Smith, M.
 The easily accessible, continuous exposure of Late
Carboniferous (Westphalian A to Early Westphalian C) Cumberland Group strata at
Joggins, Nova Scotia provides a unique opportunity to study fluvial-lacustrine
coal-bearing rocks. Previous studies have examined parts of the exposure and
presented paleoenvironmental interpretations. This study focuses on the analysis
of various types of floodplain deposits and their incipient paleosol profiles in
an attempt to reconstruct paleoenvironmental conditions for the entire exposure.
The Joggins exposure was subdivided into four parts. From the lowest and oldest
to the upper most and youngest these include: (I) Red and gray mudstone,
lacustrine limestone and thin coal layers, (II) Gray mudstones containing
abundant Stigmaria rooting and siderite nodules alternating with thick,
coal deposits, (III) Red mudstone containing abundant gray mottles, roots and
calcite nodules and separated by thin black, carbonaceous mudstone layers and
(IV) poorly developed coal seams, gray mudstone containing Stigmaria rooting
and siderite nodules interbedded with red mudstone layers. A floodplain model
for this entire exposure consists of six parts: (1) active river channel, (2)
channel margin, (3) proximal floodplain, (4) intermediate floodplain, (5) distal
floodplain and (6) lacustrine conditions. Four typical paleosol profiles
describe changing drainage conditions across the floodplain by the transition
from red to gray mudstone and the establishment of a forested peatland in the
intermediate floodplain. The presence of numerous, superimposed, incipient,
sandy paleosols in the channel margin followed by the occurrance of fewer,
thicker, organic rich, mudstone paleosols in the intermediate and distal
floodplain describe a trend of increasing surface stability away from the active
river channel as defined by a reduction in the frequency and capacity of channel
flooding events. Different floodplain conditions are found to predominate in the
four parts of the exposure: (I) lacustrine conditions with adjacent, well
drained red mudstone and isolated peat accumulation, (II) extensive peatland
development, (III) well drained oxidizing conditions and the reoccurrance of red
mudstone layers and (IV) floodplain conditions intermediate between parts (II)
and (III). A transition from humid conditions in part (II) to semi-arid in part
(III) suggest a change in climate possibly influenced by tectonic rejuvination
of upland source areas. Micromorphology reveals an abundance of features which
have benn arranged into microsequences and micro-facies. These describe
progressive pedogenesis throughout the paleosol profiles. Of continuing interest
are the calcite tubes and gray claystone pedotubules surrounded by Fe oxide
layers or accumulations of Fe-rich micro-nodules and micro-concretions. The
underclay profiles contain chlorite throughout the profile and illite weathering
and an increased kaolinite content near the top of the profile. These features
describe the pedogenic processes and possible effects of diagenetic alteration
which effect profile development and preservation. The lack of significant
weathering trends revealed by the geochemical analysis substantiates the
incipient nature of these profiles.
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