Lewis High Sediment Supply and Rising Clinoform Trajectories Indicate Highstand Fan Generation

Carvajal, Cristian R.¹, Ron Steel¹ (1) The University of Texas at Austin, Austin, TX

The high-supply Lewis system challenges the conventional idea that high volumes of sand are bypassed to the basin floor mainly at times of relative sea-level lowstand. Abundant Lewis sandy deepwater slope and basin-floor deposits occur in clinoforms exhibiting both a flat and rising shelf-edge trajectory. In the former case, fan thickness ranges from 90 to 107 m and fan area between 2212 and 2580 km2. In the latter case, fan maximum thickness is less than 76 m and fan area ranges between 1387 to 2234 km2. Flat trajectory clinothems, more akin to the lowstand model, therefore contain the highest volumes of sand. Nonetheless rising trajectory clinothems, indicating a rising relative sea-level regime, contain significant deepwater deposits too. We suggest that these are highstand fans.

The high supply character of the Lewis is key to the occurrence of high sand volumes at highstand times. High supply is indicated by the Lewis aggradation and accretion rates of 267 m/Ma and 42 km/Ma respectively (conservative estimates). These rates are much greater than rates on other margins of similar water depth (< 1,000 m) as the Spitsbergen, Porcupine, North Slope, NW Australia and South Africa margins. In these margins, accretion rates range up to 16 km/Ma, and aggradation rates up to 200 m/Ma, denoting them as low supply systems. The voluminous Lewis deepwater sands thus provide a spectacular example of the highstand end member of deepwater fan generation.

AAPG Search and Discovery Article #90055©2006 AAPG Rocky Mountain Section Meeting, Billings, Montana