Reconstruction of the Leduc and Wabamun Rock Salts, Youngstown Area, Alberta

Abstract

There are four relatively thick (>20 m) bedded Devonian rock salt units in the Youngstown area, south-central Alberta, Canada (T25–35, R5–20W4M), those of the Cold Lake Formation, Prairie Formation, Wabamun Group, and Leduc Formation. There is no substantive evidence that the former two Middle Devonian rock salts (Cold Lake and Prairie) have been leached post-depositionally in the study area. However, the latter two Upper Devonian rock salts (Leduc and Wabamun) have been leached extensively in places, and are preserved now as irregularly shaped bodies of variable areal extent, having maximum net thicknesses on the order of 45 m and 40 m, respectively.

The dissolution of the Wabamun and Leduc rock salts has caused the overlying strata to subside, more-or-less on a one-to-one basis. As a result, at any well location in the study area there is a direct correlation between the relative subsidence of a specific geologic horizon and the net thickness of all rock salt dissolved after the deposition of that horizon. This direct relationship between dissolution and subsidence is the key to reconstructing the paleodistribution of the Wabamun and Leduc rock salts and determining the timing and mechanisms of leaching.

In this paper, a suite of present-day and original net-thickness maps for the Leduc and Wabamun rock salts in the Youngstown area of Albert are presented. These maps were generated using well-log control. They are based mostly on the interval thicknesses of the residual rock salts and the encompassing Wabamun and Leduc intervals, and variations in structural relief at the Wabamun and Leduc levels and along the overlying post-Devonian horizons. Interpretation of the maps of net salt thickness supports the thesis that salt dissolution in the study area was initiated or enhanced by some or all of four principal processes: (1) the near-surface exposure of the rock salt, which resulted from the erosion of the overlying Paleozoic sediment during the pre-Cretaceous haitus; (2) the partial dissolution of underlying rock salt; (3) regional faulting or fracturing during the mid-Late Cretaceous; and (4) glacial loading and unloading.

In support of the present-day net-salt-thickness maps, two seismic profiles also are presented. The first line images an isolated remnant of Wabamun rock salt; the second crosses a linear Wabamun salt-dissolution feature. Leaching at the second location was initiated during the mid-Late Cretaceous, probably as a result of faulting or fracturing.