CONCHOSTRACANS AND OSTRACODS IN REDBEDS OF THE GETTYSBURG FORMATION (SOUTH-CENTRAL PENNSYLVANIA) -- EVIDENCE FOR LACUSTRINE ORIGIN OF AT LEAST SOME TRIASSIC REDBEDS

Initially, in eastern North America, the Triassic (into early Jurassic; Newark Supergroup) redbeds were interpreted as flood-plain deposits under tropical, seasonal wet-dry conditions. In the Gettysburg Basin in south-central Pennsylvania, except for a few dinosaur footprints at scattered localities, these redbeds lack animal fossils. In other Triassic basins, black shales (missing near Gettysburg) were long ago recognized as lake or playa sediments, from their fish and invertebrate fossils; the Lockatong shales in the Newark Basin proper have been the most intensively studied. More recently, the redbeds there have also been suggested to be lake sediments, on stratigraphic/sedimentologic grounds.

Further evidence for lacustrine origin of at least some Gettysburg-Basin redbeds are newly-discovered fossil conchostracans (clam-shrimp) and ostracods (bean-shrimp), in red medium-bedded siltstone in the middle horizons of the Gettysburg Formation, of mid-Norian age (~210 Ma), southeast of Harrisburg. The locality is an abandoned clay pit, 0.5 mi (0.8 km) N110ºE of the bridge carrying Grubb Street over Swatara Creek on the north side of Royalton (Middletown 7.5' quad.; Dauphin Co., PA).

Several specimens are black, 1.5 mm diameter, oval to nearly circular, low-domed, smooth-surfaced (no obvious growth lines), with a thin marginal flange, and variably broken or dented at the apex (diagenetic compaction). These can be identified as the conchostracan Palaeolimnadia? sp. indet. (Olsen '88 Devel Geotect 22(A):202-F).

At first glance, these conchostracans might be mistaken for megaspores from land plants (lycopods or ferns). However, such palynomorphs do not occur in redbeds (their sporopollenin walls are destroyed by oxygenation during deposition or diagenesis), and most such exhibit a trilete scar (not seen on these specimens).

Much rarer are minute (0.3-mm diameter) black dots, identifiable as Darwinula? sp. indet.; these are simple smooth-surfaced ovoid ostracods (Swain & Brown ‘72 USGS Prof Pap 795: pl. 1).

If correctly identified, these arthropods indicate deposition under aquatic/submerged, relatively fresh-water, shallow or near-shore lake, pond, playa, or shore-line conditions, as opposed to subaerial, fluvial flood-plain circumstances, thus confirming lacustrine origin of at least some of the Gettysburg redbeds.