The Claiborne Group (Eocene) in North East Texas consists of clayey sandstones and mudrocks, both with variable proportions of dark green to brown clay pellets deposited in a marginal to open marine setting on the Gulf Coast margin. The composition of concretionary carbonate and clay pellets, from 5 localities, has been investigated using a wide variety of analytical techniques. Our palaeontological, sedimentological, C/S and stable isotope data support an interpretation of marginal marine facies with limited freshwater input, for the Claiborne Group. Dark green (mature) pellets are predominantly associated with transgressive, bioturbated, marine sediments. From the Weches Formation through to the youngest Crockett Formation there is a trend from pellets composed mostly of dark green serpentine-rich mixed layer clay to those of mostly brown (immature) nontronite-rich mixed layer clay. The serpentine is intermediate between odinite and berthierine and may have been originally odinite that has undergone partial transformation to berthierine. Pellet maturity apparently corresponds with the degree of winnowing and sedimentation rate. It does not appear to correspond with global temperature fluctuations, though the serpentine-rich rather than glauconite-rich composition may do. Siderite cement, mostly in the form of concretions, post-dates the iron-rich clay pellets and pre-dates calcite cement. Carbonate and apatite concretions show a wide range of mineral cement textures, reflecting variation in sedimentary environment and early diagenetic processes. Based on water temperatures derived from verneriacardia bivalves a precipitation temperature of 20 °C is assumed for the concretionary siderite. Using this temperature we surmise that the siderite precipitated from marine pore waters with intermittent meteoric water mixing.