Sedimentology of a Tempestite: Episodic Deposition in the Cardium Formation, Pembina Oilfield Area, West Central Alberta

Abstract Deposits of the Cardium Formation in the Pembina oilfield area comprise five lithofacies: 1. bioturbated, dark grey shale; 2. bioturbated, thin- to very thin-bedded, fine-grained shale, siltstone and fine-grained sandstone; 3. thin- to very thin-bedded, fine-grained sandstone, siltstone and black shale; 4. medium-to thick-bedded, very fine- to fine-grained sandstone; and 5. conglomerate. All five lithofacies reflect a marine depositional environment, corroborated by the characteristic sublittoral to bathyal trace fossil fauna. Deposits of lithofacies 5 and the underlying lithofacies 4 are not contemporaneous, although they accumulated in a similar environment. The conglomerate is in contact with all the other lithofacies and incorporates shale, siderite and sandstone clasts. In contrast, chert granules and pebbles, characteristic of lithofacies 5, are extremely rare in the underlying lithofacies, except near the contact with lithofacies 4, where mixing through bioturbation has occurred. The sedimentary sequence is characterized by a transitional interval from bioturbated beds with decreasing shale content upward to sandstone deposits with parallel lamination as follows: low-angle inclined stratification; (laser, lenticular and wavy bedding; scour and fill structure; load casting; graded bedding; in-phase, climbing, convex and truncated-crest ripples; lithoclasts of surrounding lithologies; and destruction and truncation by scour of bioturbation traces, in sandstones and conglomerates. Capping conglomerates are, generally, poorly sorted and commonly contain shale as drapes and interbeds. Evidence for physical and biological reworking of these deposits is abundant and is distributed discretely, suggesting that contrasting energy regimes affected the overlying water column. Biological reworking of these rocks would have occurred during periods of low water agitation and turbulence. Conversely, recurring and frequent physical reworking would have taken place during periods of high agitation and turbulence. These contrasting responses are the product of sedimentation below fair-weather wave-base and above storm-weather wave-base.