Lithofacies variability in fine-grained sedimentary successions is poorly known relative to that present in other sedimentary strata. In most existing models, researchers have argued that the mechanisms that underpin facies variability are produced by a combination of detrital inputs to the basin, primary production in the water column and diagenesis once the grains have been produced. In order to investigate the validity of these models, a fine-grained, mudstone succession (Cleveland Basin) exposed on the Yorkshire Coast of NE England has been investigated using a combination of field, optical, electron optical, and geochemical methods. Analyses reveal that there is a great deal of cryptic lithofacies variability preserved on <10-2 m scale in these strata. Individual depositional beds are very thin (<10-2 m), commonly upward-fining, exhibit triplet fabrics and low angle ripple lamination, contain widely differing proportions of both detrital and productivity derived components, and have been subjected to varying bioturbation and early diagenesis. Existing models used to explain lithofacies variability underestimate the episodic and dynamism character of this sedimentary system.