Page 218 - Carbonate Facies in Geologic History

Y oredale Cycles 205

Shale YOREOAlE CYCLES IN
'COm,"
~ 'OW""O'

MiddleSkoterow lsi

....
w
U
Pockstone .g rains tone >-
U
Block laminaled argil. lsi
Brachiopods and zoophycos
Clodoconu. and
large produclids lower Skaterow lst
~~~~Coal
Seat earth. underclay
Ferrug inous stoins
~-::S"""'T=" ~gmoria roollets

....
w
U
>-
U
/ Upper Longcraig lst

Crinoidal 1st, Fig. VIl-3. Lithologic details of two
/ dark gray and argil.
-",:-r-,,.,"" Coo I y oredale cycles from the Scottish Low-
Seat earth Middle longcraig lst lands, Firth coast east of Edinburg, near
~--n>.. lithostrotion
Dunbar

of the midcontinent Pennsylvanian cyclothem and can be similarly interpreted.
The transgression represented by the limestone preserves the underlying rotting
vegetation as coal.
Several theories have been proposed to account for the regularity of the
Yoredale cyclothems. These include (1) a tectonic hypothesis of shelf-tilting (Bott
and Johnson, 1967), (2) an idea of periodic compaction of argillaceous and or-
ganic matter under a requisite load of sediment (Westol1, 1962), and (3) what is
perhaps a simpler sedimentologic explanation, the steady subsidence of the shelf
with periodic ingress of terrigenous material caused by shifting of distant deltaic
distributaries (Moore, 1959) (Fig. VII-4).
Other cyclic patterns resembling those of the Y oredales are known. In al1 of
these, marine transgression occurs under nonterrigenous conditions and a dis-
tinct, wel1-defined carbonate marker bed with a sharp lower contact is formed. A
very similar model to the Yoredale cycles is described by Van Siclen (1972) and
Gal10way and Brown (1973) from north central Texas Pennsylvanian strata.
Furthermore, a similar cyclic pattern abounds in essential1y terrigenous sedi-
ments. In such cases the upper clastic unit is much thicker than the transgressive