Petrographic and stable isotopic analyses of stromatolitic sediments deposited in nearshore environments provides us with some of the best information available on ancient environments. Diamond drill hole CAR 58 penetrated 110 m of sediments in the lowermost part of the Proterozoic (probably Helikian age) Carswell Formation of northern Saskatchewan and gave us such an opportunity. The rocks are mainly dolostone and include, in descending order of abundance, cyanobacterial laminites, stromatolites, dolomicrites, dolorudites, breccias, and oolites. Stromatolites and Cyanobacterial laminites increase in abundance up-section, and deposition is interpreted as having taken place in conditions of increasingly restricted water circulation through time. The carbon isotope ratios vary from about -0.5 to -IS%, (Pee Dee Belemnite (PDB)) in the section except near the base where they assume values near -2.5%,. The oxygen isotope ratios (vs. PDB) increase from about -9.3%, at the base to -7%, at the top, with anomolously high values, more positive than -7%,, at two positions in the sequence. Original depositional structures and textures are still visible in most of the rocks, but gypsum has been replaced by dolomite, there has been some silicification, and original features have been obliterated by dolomite rhombs in a few samples. The upward trend to less-negative values of the oxygen isotope ratios is interpreted in terms of changing depositional environment involving a deepening but more protected basin, with increased evaporational concentration of the heavier isotope. Scatter diagrams of carbon and oxygen isotope ratios place the Carswell Formation dolomites close to the mainstream of other Proterozoic stromatolites but indicating some evaporative alterations during deposition.Les analyses pktrographiques et des isotopes stables de dkpdts de skdiments stromatolitiques dans des environnements de littoral fournissent, entre autres, des enseignements trks prkcieux sur les anciens milieux. Le forage au diamant CAR 58 d'une profondeur de 110 m dans les sdiments de la base de la Formation de Carswell, du ProtkrozoYque (probablement d'&ge hklikien), dans le nord de la Saskatchewan nous a donne une telle opportunitk. Les roches dominantes sont des dolomies, incluant dans un ordre dkcroissant d'abondance des laminites cyanobactkriennes, stromatolites, dolomicrites, dolorudites, brkches et oolites. La frkquence des stromatolites et des laminites cyanobactkriennes augmente en montant dans la sequence, et nous croyons que les conditions de skdimentation ktaient accompagnkes d'une restriction graduelle de la circulation de l'eau avec le temps. Les rapports des isotopes du carbone varient d'environ -0,5 B -1,5%, (<< Pee Dee Belemnite * (PDB)) au travers la coupe, sauf p r b de la base ou les valeurs se rapprochent de -2,5%,. Les rapports des isotopes de l'oxygkne (vs. PDB) croissent d'environ -9,3%, ti la base ti -7%, au sornrnet, avec des valeurs anomalement klevkes, et plus positives que -7%,, B deux endroits dans la s6quence. Les st...