Page 219 - Carbonate Facies in Geologic History

206 Late Paleozoic Terrigenous-Carbonate Shelf Cycles

South North
COAL BElT CYCLOTHEMS
OFFSHORE FACIES YOREDAlE FACIES NEARSHORE FACIES

Zone ~ Zone 3 Zone 2 Zone I
limestone Shale and limestone Cool, sandstone, Coal , sandstone, Hinterland
shale and limeston shale
Cool

Fig. VII-4. Stratigraphic model showing variation in facies from basin to shore (south to
north) in a single Yoredale cyclothem. Vertical scale and dip on deltaic front very exagger-
ated. Illustration after G.A.L. Johnson (1961)

unit and tends to be deltaic; an upward shoaling clastic sequence is typical.
Examples include the Eocene cycles of the Texas Gulf Coastal plain (Bornhauser,
1947; Fisher, 1964) which contain a thin, shelly greensand widely transgressive
over depositional surfaces and a regressive phase consisting of thick, prograding
deltaic sand wedges. In the Lower Triassic Moenkopi Group (Virgin Formation)
of southwestern Utah (Poborski, 1954) several marine transgressions are recorded
during periods of no detrital influx. They are represented by thin, open marine
limestones, overlain by thicker gypsiferous shales and siltstones. The classic Mid-
dle Devonian facies pattern of the Appalachians consists of thin limestones suc-
ceeded regularly by regressive deltaic facies and so follows the Y oredale pattern
(Cooper, 1957). The Rocky Mountain Cretaceous contains many regressive sand-
stone wedges progradational from the rising cordillera eastward into the Pierre-
Mancos shale basin of the Midcontinent. The transgressive phases of these cycles
are relatively thin westward-projecting shale tongues with a few calcareous con-
cretions (Young, 1955).

Pennsylvanian and Wolfcampian Shelf Cyclothems
of the Midcontinent and Southern Rocky Mountains

Well known cyclothems of mixed clastic and carbonate sediments are also general
throughout North America during the Late Paleozoic although the sequence of
events recorded by them differs slightly from that of the Y oredales. They have
been described by many writers, beginning with Charles Udden (1912), from
outcrops more or less parallel to strike from the midcontinent states of Nebraska,
Iowa, Kansas, Oklahoma, and north central Texas (Fig. VII-5). The algal plate
mud mounds of the Pennsylvanian shelves, described in the preceeding chapter,