DEPOSITIONAL DYNAMICS IN A MIXED CARBONATE-SILICICLASTIC SYSTEM: MIDDLE/UPPER CAMBRIAN ABRIGO FORMATION, SOUTHEASTERN ARIZONA

Sedimentary processes recorded in the mixed carbonate–siliciclastic systems of Cambrian inner- and mid-shelf settings are relatively poorly understood with respect to modern facies interpretation. The Abrigo Formation of southeastern Arizona, for example, hitherto studied only in a reconnaissance manner 50 years ago and interpreted then as peritidal, provides an instructive opportunity for a new appreciation.

High-energy transgressive shelf sandstones are abruptly overlain by interbedded carbonate and siliciclastic facies that comprise an overall shallowing-upward succession. Eight facies associations are recognized: (1) thin-bedded nodular lime mudstones bioturbated by Planolites with sporadic lenses of intraclastic rudstone; (2) laminated mudstone or lime mudstone occasionally intercalated with lenticular-bedded siltstones and small-scale hummocky cross-stratified fine-grained sandstones; (3) laminated mudstone or lime mudstone interbedded with small-scale hummocky cross-stratified, fine-grained sandstones with common gutter casts and intraclastic rudstone; (4) hummocky cross-stratified sandstone regularly interbedded with laminated mudstone or lime mudstone; (5) bioclastic grainstone and packstone, and oolitic-oncolitic packstone; (6) lime mudstone, wackestone with rare stromatolites; (7) amalgamated hummocky cross-stratified, medium-grained sandstone, locally with intraclastic rudstones; and (8) trough cross-stratified, medium-grained sandstone interbedded with thin mudstones. Lateral variation is striking: the shallowing-upward pattern is present over the whole study area, but individual beds cannot be correlated.

The depositional environment is interpreted as wave-dominated and influenced by storm processes, with lower offshore at the bottom of the succession (1 and 2), overlain in turn by upper offshore (3), offshore transition (4–6), lower-middle shoreface (7) and upper shoreface (8). Variation in the carbonate content of mudstones and deposition of limestone in individual stratigraphic sections are ascribed to varying vigor of the shallow-water carbonate factory and its lateral variation. Carbonate intraclasts owe themselves to seafloor cementation of lime mud, not to a regional change in bathymetry and storm wave-base.