Page 217 - Carbonate Facies in Geologic History

204 Late Paleozoic Terrigenous-Carbonate Shelf Cycles

~ ~5~ • N
Soatoarth
: ," .'" ..
t·· .. ··· -··>1 Sandstones 3 4 ~ ALSTON \
BLOCK
BLOCK
Shot ..
0 10 25
~ mil.s
lImo.tonos
Fig. VII-2. Large scale variations in two Yoredale cycles (IV and II) south to north across
northern England from Moore (1959, Fig. 13). The letters A through E mark traceable lime-
stone beds. Boundary of the two cycles is marked by top of terrigenous clastics below C or D.
For location of Askrigg and Alston blocks see Fig. VII-l

The clastic deposits clearly were formed as a typical outbuilding deltaic se-
quence as outlined by Moore (1959) and reviewed succinctly by Selley (1970)
(Fig. VII-2). The next overlying limestone rests with sharp contact on the coal
(Fig. VII-3). This is why the cycle is considered conventionally to begin at the
limestone base. The limestones are dark, open marine wackestones-packstones
with varied bioclasts, corals, brachiopods, bryozoans, crinoids, algal onkoids, and
even some oolite beds. Fine argillaceous seams and stylolites are present. Cross-
bedded oolite grainstones, encrinites, and small patchy "reefs" consisting of var-
ious types of organisms, are known. Chert nodules and siliceous limestone occur
in the upper parts of limestone beds. The limestone grades upward to laminated
black, pyritic, micaceous shale and siltstone with siderite concretions. Productid
brachiopods are its only common fossils.
The Y oredale cycles represent a succession of marine incursions across the
unstable shelf blocks north of the Hercynian trough. Each transgression occurred
without any terrigenous incursion. The limestone represents water of open circu-
lation and hence moderate depth over considerable shelf areas. Probably the
deepest inundation occurred during deposition of the prodeltaic black shale-
siltstone unit which heralds the clastic influx resulting from deltaic advance from
the north. The fine, rippled, cross-laminated sands and the cross-bedded, coarser
sands represent distributary and alluvial channel deposits. The coal and rootlet
beds represent delta plain swamps, capping the sequence.
The part of the sequence consisting of coal, widespread thin limestone, and
black shale-siltstone, much resembles the deposits beginning the inundative phase