Page 215 - Carbonate Facies in Geologic History

Chapter VII

Late Paleozoic Terrigenous-Carbonate Shelf Cycles

Introduction

Chapter II emphasized the importance of the study of cyclic sedimentation and
discussed the basic principles involved in its formation. Most characteristically
cyclic sediments are found in shelf areas although regular changes in sedimentary
conditions may be reflected in many types of basinal sediments (evaporites, flysch,
carbonate-shale rhythms). Cyclic shelf deposits are particularly common in the
Late Paleozoic of the northern hemisphere. The persistence of platforms situated
just at sea level, the tectonic activity which provided periodic incursions of terrig-
enous clastics, and the enhanced possibilities of eustatic sea-level fluctuations
(either glacially or tectonically induced) all coincided to make this time in earth
history favorable for cyclic sedimentation.
This chapter compares two types of shelf cycles in which terrigenous clastic
influx from distant deltaic regions is interlayered with carbonate strata: (1) the
Y oredale Lower Carboniferous of the British Isles, and (2) the Pennsylvanian
cyclothems of the midcontinent area of North America. Previous chapters have
discussed the thick pure carbonate deposits formed on the shelf margins and
basinal slopes at the same time as these cyclic sequences. The description here of
the shelf deposits is brief as an extensive literature on both sequences already
exists.
Both the Yoredale and the North American Pennsylvanian cyclothems must
have been deposited across very extensive flat shelves in tropical seas whose
carbonate deposition was periodically interrupted by clastic influx from a distant
terrigenous source. Differences in stratigraphic sequence might have resulted
(1) from differences in timing of clastic influx relative to sea-level rise and fall, or
(2) to the extent to which deltaic progradation was able to overcome subsidence.
Thus, in the Yoredale case this influx of clay and sand either forced a gradual
marine regression or coincided with an independently caused sea-level drop.
Transgression is represented by a single limestone sheet. In contrast, in many
midcontinent Pennsylvanian-Wolfcampian cycles and in related cycles in New
Mexico, marine submergence commonly took place during limestone deposition
interspersed with incursions of clay and sand. The traditional interpretation is
that terrigenous deposition mainly coincided with the marine invasion and was,
in each cycle, eventually overcome by the advancing marine conditions. This
results in an interbedded sequence oflimestone and terrigenous clastics preceding
the record of maximum transgression and obviously in a much more complicated
cycle. This explanation is founded on the interpretation of the channels below the
limestone portion of the cycles.