Abstract

The Middle Turonian Kaskapau Formation in the Rocky Mountain Foothills of northeastern British Columbia comprises up to 950 m of shallow marine sandstones cyclically interstratified with marine mudstones and minor lagoonal deposits. Transgressive surfaces are commonly mantled by thin pebble lags. A transgressive pebble lag exposed in Quality Creek near Tumbler Ridge town site includes rare, well-rounded quartzite cobbles and a quartzite boulder, which were deposited at a paleolatitude of about 67°N. Encrusting organisms are unevenly distributed over five of the largest clasts. Disciniscid brachiopods are abundant on one cobble but are otherwise rare. A conical-shelled lingulate brachiopod with concentric growth lamellae occurs sparsely, and a weakly ribbed lingulate brachiopod is represented by one shell. Serpulid worm tubes, encrusting bryozoans, and possible basal attachments of corals are also present. Attached foraminifera are abundant and distributed over several clasts. Generic diversity and taxic dominance values are moderate. The fauna includes many elements typical of Cretaceous lower-latitude hard substrates but is unusual in lacking bivalves; their absence might be due to a combination of abundant clastic supply, high turbidity, possibly lowered salinity in the nearshore zone, and relatively low water temperature (∼10°C).

The gradient of rivers that supplied gravel to the Kaskapau shoreline was estimated from clast size (typically <30 mm) and bed thickness in coeval fluvial conglomerate. Results suggest river gradients of about 0.3–0.4 m/km close to the shoreline. Calculations based on the encrusted cobbles suggest an unreasonably steep river gradient of about 1.9 m/km. The most likely alternative transport mechanism is rafting in the roots of trees, although the high paleolatitude might not preclude rafting by winter river ice.

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