The Ordovician Trenton Group (Sherman Fall and Cobourg formations) and the Lindsay (Collingwood Member) and Blue Mountain formations of southwestern Ontario were examined using Rock-Eval pyrolysis, gas chromatography, gas chromatography-mass spectrometry,and incident-light microscopy to evaluate their paleodepositional environments, thermal maturities, and source rock potential. All units contain sufficient amount of oil-prone (type II), predominantly marine organic matter to be considered as petroleum source rocks; Unstructured bituminite with varying proportions of unicellular alginite are the dominant dispersed organic matter macerals. The bituminite typically occurs in massive to laminated, granular or patchy populations that commonly show microtextural relationships. Persistent inclusions of Leiosphaeridia telalganite demonstrate that planktonic algal debris was a primary organic substrate for blooming microbes. Disseminated coccoidal Gloeocapsomorpha prisca is found in minor amounts, usually in association with common to abundant acritarchs, Zooclasts (chitinozoa, graptolites, scolecodonts)and solid bitumen also are present as maceral inclusions within the bituminite network. The biomarker distributions for all of the studied units are those expected for marine organic matter deposited in a elastic-dominated environment. The extracts are characterized by a smooth n-alkane profile, with low abundance of C20+ members, typical for marine derived organic matter. Pristane/phytane ratios range from 0.37 to 1.72, indicating dysoxic conditions during deposition. Smooth C-31-C-35 homohopane profiles, T-s/T-m ratios (typically above 1.0), and a higher concentration of diasteranes relative to regular steranes all appear to indicate the clay-bearing character of these rocks. The predominance of C-30 hopane over C-29 regular sterane is interpreted to reflect a primary microbial input and extensive reworking of the organic matter. Optical (reflectance, fluorescence) and geochemical

Keywords:GLOEOCAPSOMORPHA-PRISCA ZALESSKY;APPALACHIAN BASIN;SOUTHERNONTARIO;THERMAL MATURITY;MICHIGAN BASIN;SEDIMENTATION;REFLECTANCE;INDICATORS;USA;DIAGENESIS