TRIASSIC PHOSPHATIC ROCKS OF NORTHERN ALASKA: SPATIAL AND TEMPORAL DEPOSITIONAL PATTERNS ON A HIGH-LATITUDE, LOW-ANGLE RAMP

The Shublik Fm. (northern Alaska) was deposited on a high-latitude (~45-60°N), gently sloping ramp, and contains the largest phosphorite accumulation of Triassic age (5x10⁹ t; Cathcart, 1991). New stratigraphic, sedimentologic, and geochemical data from the Shublik in 28 wells and 15 outcrop localities provide insights into the nature and origin of these strata. Phosphate accumulated from late Early to Late Triassic (Smithian-late Norian), but highly phosphatic strata (HPS, 10-28% P₂O₅) formed chiefly during the late Middle (Ladinian) and middle Late (early-middle Norian) Triassic. Geographic foci of HPS deposition shifted with time.

Ladinian HPS constitute a petrographically distinct interval up to 7 m thick recognized across 80 km of outcrop in the NE Brooks Range and in several wells 110 km to the NW. Beds are parallel- and cross-laminated and contain transported, sand-sized phosphatic clasts—mainly simple peloids and phosphate-coated quartz grains and flat clam fragments—cemented by silica, phosphate, and/or calcite. Phosphate content and grain packing decrease upward within the interval and carbonate content increases. These strata formed on the middle ramp during transgressive and early regressive parts of a 2^nd or 3^rd order depositional cycle.

Norian HPS are more widely distributed and heterogeneous than those of Ladinian age. The Norian HPS occur through a ~80 by 180 km outcrop area, and in numerous wells on the eastern North Slope. Phosphate in these strata is both transported and in situ and is chiefly cm-scale nodules and sand-sized peloids. It is most abundant in intervals ≤ 2 m thick at the tops of regressive limestone parasequences that were deposited on the middle to inner ramp.

HPS of other ages are less common. Smithian-Anisian HPS include phosphate ± chert clast conglomerates in a few west and central N Slope wells and phosphatic nodule horizons that top siltstone parasequences in some Brooks Range outcrops. Carnian HPS that contain concentrations of phosphatic nodules and peloids occur mainly in 2 N Slope wells.

The Shublik Fm. represents part of a worldwide shift of phosphorite formation into higher latitudes during the Triassic. If, as previously suggested, HPS in the Shublik formed in response to a marine upwelling system, our data show that the effects of this system varied through space and time.