SEDIMENTOLOGY, FACIES SUCCESSIONS AND CYCLICITY OF A SECTION OF THE JOGGJNS FORMATION, JOGGINS, NOV A SCOTIA

Abstract

The Joggins cliffs near Amherst, Nova Scotia, form one of the world's premiere Carboniferous coal-bearing exposures. A 145 m section was measured in the vicinity of Bell's Brook. The section is classified into three lithofacies groups (sandstone, mudstone, and organic-rich sediments) deposited in a fluvial to coastal (delta plain) environment. Multistoried channel sandstones up to 6 m thick represent delta distributary and anastomosing channel bodies, and sheet sandstones represent crevasse splay deposits. Heterolithic sandstones are either overbank levee deposits (in part stacked crevasse splays), or formed in subaqueous settings, possibly interdistributary bays, as a result of repeated crevassing. Grey clay-rich mudstones classify as "seat earths" or Gleysol paleosols and grey platy mudstones are hydromorphic soils that experienced less vegetative activity. Red mudstones are Vertisols formed under seasonal, oxidizing conditions. Carbonaceous shales are clastic swamp deposits and are associated with coals (Histosol) formed in peat mires. A single carbonaceous, shell-rich limestone formed after a major transgressive event which flooded a peat mire. The rock types are grouped into two facies successions: wetland facies succession (coastal plain deposit) and dry/and facies succession (alluvial plain deposit). The two successions alternate up section in regular intervals between 10-4 7 m thick. The wetland succession represents an interdistributary bay fill or progradation of delta lobes in areas close to major river mouths. A distinct feature is composite units, up to 15 m thick, that commence with a flooding surface (coal, grey shale, carbonaceous shale) and coarsen upwards through grey shale to stacked sandstones with in situ lycopsid trunks and Calamites; they are capped with grey paleosols. The dryland succession represents a relative drop in groundwater levels, generating a well drained flood plain with intermittent influx of sediment in sheet flows (crevasse splays). Boundaries between the red and grey successions are gradational, and the strata comprise progradational (grey to red upwards) and retrogradational (red to grey) parasequence sets. Sequence boundaries could not be identified. The cause of cyclicity cannot be determined with certainty, but tectonic and glacioeustatic ( allocyclic) effects are implicated, coupled with climatic and autocyclic factors (delta switching or channel migration).

Key Words: lithofacies, delta plain, facies succession, cyclicity, allocyclic and autocyclic effects