ORIGIN OF FIBROUS GYPSUM IN THE NEWARK RIFT BASIN, EASTERN NORTH-AMERICA

Cores from the red beds constituting the Upper Triassic Passaic Format ion, a syn-rift sequence in the Newark Basin, display subhorizontal fr actures (veins) filled with satin spar gypsum, Such veins are apparent in the shallow subsurface, 100-300 m below present ground level, but are not observed in outcrops or in quarries because of groundwater dis solution, These fractures are roughly parallel to the present ground s urface in the basin, suggesting topographic influence on their formati on, Fractures observed in cores are seen to crosscut stratigraphic bou ndaries, indicating that the veins formed after stratal tilting and fa ulting and thereby post dare the early Late Jurassic The gypsum crysta ls making up the vein fill grew perpendicular to the fracture walls, s uggesting that opening and filling of the fractures was contemporaneou s and that the fractures are extensional. That gypsum crystallizes fro m meteoric and connate waters and is derived by dis solution of associ ated evaporites is reflected in the delta D, delta(18)O, delta(34)S, a nd Sr-87/Sr-86 isotopic compositions of the sulfate, Fractures are int erpreter! to be a result of unloading and exhumation. Fractures were i nitiated after major depositional and tectonic events took place and p ost-date the Late Jurassic, Satin spar precipitated in those fractures , carried in by gypsum-saturated brines derived either by dissolution of the nearsurface sulfate evaporites or rehydration of more deeply bu ried anhydrite, Such satin spar veins are not ''evaporites'', but are composed or chemically remobilized sulfate.