A sedimentary rock of ultrabasic composition (MgO 28%, SiO2 48%) has been identified in an Archean volcanic sequence ranging from basaltic to peridotitic komatiite in composition, in the Lac Guyer greenstone belt, James Bay region of New Quebec. The ultramafic sedimentary rock is cyclically layered with an internal stratigraphy indicative of deposition from an aqueous turbidity current. Layers which are interpreted to correlate with the arenaceous A, B, and C divisions of turbidites are pyroxene-rich and display sedimentary features such as grading, parallel and cross-laminations, and climbing ripples. Foliated layers with higher normative olivine contents preserve loading and soft sediment deformation structures indicating a pelitic nature when deposited and are interpreted as Bouma E divisions. These features combined with a sympathetic variation of Al2O3 with normative olivine content in successive Bouma divisions suggest that a Mg-chlorite with subordinate serpentine rather than olivine was present, together with pyroxenes and opaques, in the initial sediment. This sediment may have been derived from the degradation of the associated komatiitic volcanics or may represent a contemporaneous reworked ultramafic tuff or ash flow.