The Lower Cretaceous Okpikruak Formation lies unconformably on the Etivluk group in the Foothills thrust belt of the central Brooks Range. These deep marine sediments were shed northward during the first phases of Brookian deformation. The formation, as sampled, contains two petrographically distinct populations: the upper Kurupa-Oolamnagavik River drainage samples (KOR) of the Picnic Creek allochthon, and the Cobblestone Creek samples (CC) of the Endicott Mountains allochthon.

The KOR samples have an average Q:F:L of 49:21:30 and Qp:Lv:Ls of 46:37:17. Variations in total detrital grain populations are greater for

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quartz (40-50%) and rock fragments (22-39%) and less for feldspar (17-27%). The KOR population is characterized by high proportions of polycrystalline quartz and low proportions of sedimentary rock fragments. Interpretation of detrital modal analyses suggests a complex source terrane involving orogenic recycling and mixing of sediments from a magmatic arc and a subduction system.

In contrast to KOR samples, CC samples have an average Q:F:L of 16:11:73 and Qp:Lv:Ls of 11:34:55. Total grain populations vary most for rock fragments (56-85%) and quartz (6-27%) and to a lesser degree for feldspar (8-17%). The high variation in rock fragments suggests variable volcanic and sedimentary sources. Interpretation of detrital modal analyses also suggests complex sources but with a significant arc-orogen component.

Both populations are characterized by varying proportions of diagenetic products, which include albite, calcite, chlorite, sericite, and epidote.

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