Page 229 - Carbonate Facies in Geologic History

216 Late Paleozoic Terrigenous-Carbonate Shelf Cycles

Cordilleran geosyncline. They have been intensively studied in New Mexico in the
Sacramento Mountains by L. V.Cline (in Pray, 1959), Pray (1961), and Wilson
(1967 a, 1972). The author has outlined an example of cyclic and reciprocal sedi-
mentation here between the sediments of the Oro Grande basin and the narrow
carbonate shelf surrounding the eastern side of the Pedernal uplift (Fig. VII-8).
The widespread cyclic character of these strata which extend throughout the area
surrounding the cratonic blocks of the ancestral Rocky Mountains, induces, al-
most automatically, a belief in a widespread eustatic mechanism for the cyclicity.
(Even the evaporitic Paradox basin contains sedimentary cycles.) On the Sacra-
mento Mountain Pedernal shelf, however, close scrutiny of the section and its
paleotectonic history indicate that a reasonable explanation may lie in the peri-
odic uptilting and subsequent downfaulting of the tectonically unstable edges of
the Pedernal block. In this area practically all cycles show that terrigenous influx
occurred as sea level tended to rise. As in the traditional interpretation of the
Kansas megacyclothems, the cycles begin with transgressive terrigenous clastics.
Soon after clastic influx ceased, sea level reached its maximum. When the seas
cleared, carbonate accretion, perhaps with continued rise of the shelf, caused a
retreat of the sea off the shelf into the basin. Here again the timing of clastic influx
relative to sea-level fluctuation constitutes the basic control over the sedimentary
pattern.
The widespread cyclicity so evident in Late Paleozoic strata may be a conse-
quence of some eustatic sea-level changes whose effects are enhanced by local
tectonics which caused unstable shelves and periodic influx of terrigenous clastics
which shut down the carbonate production and rapidly altered the depositional
environments.