Paper No. 11
Presentation Time: 9:00 AM-6:30 PM


LABAJ, Marcelina, Central European Petroleum Ltd., Calgary, AB T2P 3G3, Canada and PRATT, Brian R., Dept. of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, SK S7N 5E2, Canada,

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.