Abstract: Alkaline Authigenic Minerals in the Early Mesozoic Newark Basin, NY, NJ, PA: Evidence for Late Fluid Movement through Mudstones

Joseph P. Smoot, Bruce M. Simonson

The Newark basin contains nearly 6 km of late Triassic to early Jurassic lacustrine mudstones deposited in a hydrologically closed basin. Authigenic alkaline minerals in cyclic lacustrine mudstones of the Lockatong and Passaic Formations have been assumed to represent depositional conditions in a saline lake or playa setting. Depositional facies change radically over a few meters to several hundred meters vertically, however, and represent deep to shallow perennial lakes, some with evaporites; shallow intermittent lakes; saline mudflats; dry playa mudflats; and vegetated mudflats both with and without saline soil evaporites. Preliminary petrographic analysis indicates the following authigenic mineral paragenesis within depositional voids such as vesicles, cracks and root structures: ( ) analcime or albite, (2) K-feldspar

(also replaces analcime), (3) dolomite (commonly replaces pre-existing silicate minerals), and (4) calcite.

Minor authigenic minerals include (1) barite that post-dates calcite, (2) base-metal sulfides that post-date dolomite and predate calcite, and (3) gypsum that postdates calcite.

The paragenetic sequence is independent of the depositional facies, except that analcime is the initial void-filling phase in the Lockatong whereas albite is the initial phase in the Passaic, roughly coincident with the first appearance of root-disrupted fabrics. The same general authigenic assemblages also fill brittle fractures and some evaporite pseudomorphs (although more commonly the pseudomorphs are replaced only by calcite). In the uppermost Passaic Formation, calcite and gypsum dominate. This distribution of authigenic minerals indicates that they are not syndepositional; argon-argon radiometric dates of the K-spar suggest it formed about 200 my ago (10-30 my after deposition). We therefore conclude that fluids moved through the mudstones long after deposition and that mineral assemblages in mudstones are not necessarily indicators of the depositional environment.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994