Based on mineralogical studies of clay-sized material, two sedimentary environments have been interpreted for the Tertiary sediments on northeastern Grand Banks of Newfoundland: (i) an acidic leaching environment (pH < 7.0) is observed as a weathered, probably desiccated crust directly underlying the Tertiary–Quaternary unconformity; and (ii) an alkaline marine environment directly underlies the weathered zone. The weathered zone has been sampled at depths of 2–3 m (locally) below the seabed surface in the study area. Mineralogically the < 2 μm size fraction is characterized by relatively high kaolinite concentrations (7 – 18%) and a clinochlore component. The more deeply buried sediments have been deposited under marginal to normal marine conditions (a regressive phase, pH 7–8). Clay-sized components in this zone are characterized by high concentrations of expandable minerals (average 45%) and chamosite.Sediments from both of these environments differ from the thin, clay-poor Late Pleistocene – Holocene sands and gravels at the present seabed surface. Late Pleistocene – Holocene sediments have clay-sized mineralogical suites, partially reworked from weathered zone components (up to 15% kaolinite, locally) but are mostly derived from a northern latitude provenance (high illite (average 28%), quartz (average 14%), and feldspar (average 19%)). Expandable mineral phases range from 0 to 24%.The differing clay-sized mineralogical signatures indicate (i) alteration of the Tertiary marine sediments (expandable minerals) by acidic leaching to form a kaolinite-enriched weathered zone; (ii) the reworking of weathered zone materials during deglacial marine transgressions; (iii) the incorporation of sediments from the weathered zone into the Holocene sedimentary regime during Recent times, possibly by ice scouring; (iv) input from northern latitude provenances by reworking of earlier deposited glacial material; and (v) minor clay input to the Late Pleistocene – Holocene sands and gravels from ice rafting over the past 10 000 years.