LOWER CAMBRIAN GRAND CYCLES REVISITED: IMPLICATIONS FOR THE INTERPRETATION OF MIXED CARBONATE-SILICICLASTIC SYSTEMS

‘Grand Cycles’ are depositional units comprised of a siliciclastic half-cycle overlain by a carbonate half-cycle, forming a conformable succession bound by sharp contacts. These sharp contacts are typically regarded as representing disconformities due to the abruptness between underlying carbonate and overlying siliciclastics, while the transition between the underlying siliciclastic and overlying carbonate half-cycles is gradational. It is generally thought that the shift, although gradual, between siliciclastic and carbonate half-cycles represents maximum flooding and it is usually interpreted as a fundamental change in the depositional system from a siliciclastic shelf to a carbonate shelf or platform. This study examines the depositional environments and sequence stratigraphy of Grand Cycles from the Lower Cambrian of the White-Inyo Mountains, east-central California.

The Precambrian-Cambrian transition in the southern Great Basin is a mixed carbonate-siliciclastic succession and has been conventionally interpreted as a carbonate platform with nearshore siliciclastic deposition and offshore carbonate deposition. Recent research, however, suggests that the converse may be more representative with nearshore carbonates and offshore siliciclastics of a continental shelf. This reversal, if proven accurate, has implications for the origins of Lower Cambrian Grand Cycles of the southern Great Basin.

Detailed sedimentological analysis of the transition between siliciclastic and carbonate half-cycles reveals that underlying siliciclastic facies represent shallow shelf to shoreface environments, while carbonate facies are interpreted as tidal inlet, tidal delta, and lagoonal environments. The transition between siliciclastic and carbonate facies is one of progradation, with carbonate facies typically overlying sandstones of the lower shoreface. Also, the presence of carbonate-capped shoreface sandstones within siliciclastic half-cycles suggests that the two types of facies coexisted in time. It also suggests that the carbonate half-cycle is the marginal marine equivalent to the shoreface and shelf siliciclastic half-cycle and that Grand Cycles represent facies shifts rather than a fundamental change in lithology within the depositional system.