ABSTRACT

The stable-shelf succession of the Middle Cambrian to Middle Ordovician of the southern Rockies consists of a limited number of rock-types, predominantly of very shallow-water origin, each of which is repeated many times throughout the succession in a cyclic manner. Eight asymmetrical depositional cycles are represented, each comprising 300 to 2,000 feet of strata and two or more fossil zones. Each of the cycles commences at an abrupt basal contact, and consists of a lower, shaly half-cycle gradationally overlain by a carbonate half-cycle. These cycles in the stratigraphic succession reflect lateral shifts in the positions of the inner detrital, middle carbonate, and outer detrital facies belts. All formational contacts are drawn at the beginnings or mid-points of depositional cycles.

Rhythmic repetitions of sequences of beds occur within both shaly and carbonate half-cycles, at an estimated frequency of 20 to 50 per cycle. The sub-cycles vary in character according to stratigraphic and geographic position, but all may be reasonably interpreted as resulting from a sudden increase in water depth and supply of terrigenous sediment, followed by gradual shoaling in clearing waters. The major cycle is interpreted as reflecting a similar sequence of environments, occurring over a much longer period.

Any hypothesis constructed to explain the cyclic deposition must take into account the following:

1. Cycles and sub-cycles reflect sequences of events parallel in character but different in magnitude and duration.
2. Large areas of the shelf are affected simultaneously.
3. Uplift in the source area, manifested by an increased supply of detritus, coincides with subsidence in the area of deposition, manifested by deeper-water lithofacies.

A "tilting-craton theory" has been constructed to satisfy the above requirements.