ABSTRACT

The Tribes Hill Formation (Lower Ordovician) of the Mohawk Valley, New York, was deposited within a broad, shallow epicontinental sea. Ten lithofacies were deposited in three distinct depositional environments: supratidal, intertidal, and subtidal. The lithofacies and the environments in which they were deposited are:

Lithofacies                                  Environment
10. Biointramicrite and biomicrite           Subtidal
 9. Oobiosparite and biointraoosparite       Subtidal
 8. Intrasparite and biointrasparite         Intertidal
 7. Mottled dolomitic micrite and biomicrite Intertidal
 6. Dolostone                                Supratidal
 5. Shale stringers and layers               Intertidal to Supratidal (Cycles)
 4. Pebble conglomerate                      Intertidal to Supratidal (Cycles)
 3. Feldspathic pelmicrite                   Intertidal to Supratidal (Cycles)
 2. Laminated feldspathic dolomite           Supratidal
 1. Mottled feldspathic dolomite             Supratidal

The supratidal sediments are composed of mottled or laminated dolomite which tends to be feldspathic, contains bituminous material, and shows birdseye textures and undulating stromatolitic structures. Fossils are absent. The intertidal sediments can be differentiated into tidal flat and tidal channel facies. The tidal flat sediments are characterized by pelletal and skeletal mudstone and pebble conglomerate. Mud cracks, lumpy structures, scour-and-fill structures, crossbedding, and erosional breaks with shale stringers are common. Fossil fragments are abundant in pebble conglomerate; sporadic birdseye textures are noted; one of the lithofacies shows abundant mottling with the burrows infilled by dolomite. The tidal flat sediments are cut by tidal channels, he lower contact of the channels being one of truncation. Blocks of tidal flat facies foundered in the tidal channels. The subtidal sediments are represented by well-bedded lime mudstones and calcarenites with ooids, abundant skeletal fragments, and intraclasts.

Deposition of the carbonate sediments began on sea-marginal supratidal flats (lithofacies 1 and 2) in which dolomite was formed by "capillary concentration" and in which a high activity ratio of alkali to hydrogen ions produced authigenic feldspar. Gentle subsidence gave rise to tidal flats (lithofacies 3, 4, and 5) which make up the most extensive environment. Pelletal mud was dried and hardened, broken into mud cracks, and the polygons of the mud cracks were incorporated in the sediments to form the pebble conglomerate. Emergence again gave rise to a supratidal setting followed by subsidence to form broad tidal flats which were dissected by tidal channels. The terminal event was subsidence to a subtidal level in which ooids and abundant fossils characterized the sediments.