ANATOMY OF A SEMIARID COASTAL SYSTEM: THE UPPER CARNIAN OF LOMBARDY (ITALY)

The mixed terrigenous-carbonate-evaporitic S.Giovanni Bianco Formation and dolomitic Campolungo Tongue (upper part of the Breno Formation), generally 2OO to 3OO m thick, are assigned to the Late Carnian. They respectively overlie lagoonal limestones (Gorno Fm.) and peritidal carbonates (Annunciata Member of the Breno Fm.), and underlie intraformational breccias and recrystallized limestones (Castro Fm.). Recognition of an unconformity, ascribed to a relative fall of sea-level (sequence boundary), allowed us to subdivide the Upper Carnian succession into two parts. ln the lower part (SGBL), six lithosomes were recognized. Red to green alluvial clastics in the south-east and south-west pass northward to mixed terrigenous-carbonate coastal sediments and finally to dolostones deposited in carbonate tidal flats. In the proximal sections of the Brescia Prealps, renewed north-westward progradation of alluvial redbeds with intercalated calclithite conglomerates points to a stage of teconic uplift. A distinct increase in quartz, representing a regional petrographic marker followed all across Lombardy, indicates deepening of erosion into the metamorphic wallrocks of the volcanic belt. A major hiatus at the top of the SGB1 is best documented in the northern Presolana area by a silcrete crust directly overlying the Julian Annunciata Member of the Breno Formation. In the Brembana Valley area, the discontinuity occurs within a greenish siliciclastic coastal plain succession, and may be traced at the top of a marker interval of interbedded reddish siltstones and sandstones. The upper part (SGB2) consists of four lithosomes. Greenish sandstones and siltstones, accumulating in coastal plains in the south-west, passed northward to mudrocks and dolostones. In the southernmost Camonica Valley area, mudrocks are locally interbedded with calcarenites containing bored or pedogenized lithoclasts ripped from the underlying sequence and varied bioclasts, restifiying to relatively open shallow-marine conditions during transgression. Next, thick gypsum accumulated in coasral salinas barred by locally oolitic platform carbonates to the north. Rare sandstone lenses occurring in the Brembana Valley at the top of the unit conrain exclusive rhyolitic detritus, indicating either a terminal phase of explosive volcanism or erosion of older felsic volcanic products.