Precambrian (> 0.54 billion years or b.y. old) rocks underlie all of Saskatchewan, but in the southern two-thirds of the province they are covered by younger Phanerozoic rocks. The exposed Precambrian Shield rocks include several Archean (> 2.5 b.y. old) continental landmasses (cratons) and younger, Paleoproterozoic (2.5-1.6 b.y.) orogenic belts separating them. The cratons are thought to be the precursors of our modern continents and probably moved about the Earth's surface on tectonic plates, much as continents do today. The orogenic belts represent zones along which two or more tectonic plates were amalgamated. They are analogous to the Himalayas, where India collided with Asia, and would have been mountain belts for tens of millions of years following the collisions. However, over time such belts in Saskatchewan have been worn down by erosion. The most obvious example is the Trans-Hudson Orogen, which occupies the southeastern part of the Precambrian Shield. It was a complex orogeny that resulted from the amalgamation of three Archean cratons between 1.9 and 1.8 b.y. ago. These were: the Rae-Hearne (Churchill) Craton, which occupies almost all of the remainder of Saskatchewan's exposed Shield; the Sask Craton, which is exposed only in very small areas west of La Ronge and in the Pelican Narrows area; and the Superior Craton, which is mainly exposed in Ontario and Quebec but may extend into the Kamsack-Carnduff area under Phanerozoic cover. The Thelon-Taltson Orogen affected only the northwestern corner of Saskatchewan; it resulted from 2.0-1.9 b.y.-old amalgamations of the Slave Craton, located in the northwest of the Northwest Territories, and from smaller cratonic slices extending into northern Alberta.
Each craton developed independently before amalgamation, but afterwards its geological history was shared with its new neighbour. This development likely included earlier cratonic amalgamations and breakups extending back to about 2.7 b.y. ago, before which the young, hot planet may have used other ways to cool. The Rae-Hearne Craton extends southward under Phanerozoic cover rocks almost to the American border, northeastward to the Arctic Islands and beyond, and is bounded by the Thelon-Taltson and Trans-Hudson orogens; granitoid rocks as old as 3.1 b.y. north of Lake Athabasca record the earliest known phase of its history. Cycles of widespread volcanism, plutonism, and sedimentation at about 2.7 b.y. and 2.6 b.y. probably record early collisional events as the craton was being assembled. Granites and sedimentary rocks 2.3 b.y. old concentrated in the extreme northwest may record the accretion of small cratonic slivers to the western margin of the Rae-Hearne Craton. 2.0-1.93 b.y.-old granitoid emplaced northwest of Lake Athabasca and northeast of La Loche resulted from consumption of an oceanic plate west of the craton. Closure of that ocean occurred when the Slave Craton and a number of smaller cratonic slivers that were also riding on the approaching ocean plate collided with the Rae-Hearne Craton. The resulting Taltson-Thelon Orogen involved major mountain building, associated metamorphism, and granite emplacement throughout the northwest between 1.93 and 1.90 b.y. ago.
The Sask Craton underlies virtually the entire Trans-Hudson Orogen and extends southward to the vicinity of the American border, but is poorly understood because of its limited exposure. The early history is recorded by sedimentary and 3.1 b.y.-old granitic gneisses, some of which carry hints of an otherwise unrecognized 2.7 b.y. ago tectonic event. A widespread 2.53-2.43 b.y.-old igneous suite may have resulted from the breaking off of the Sask Craton from a larger pre-existing cratonic body. Volcanic and granitoid rocks dominate the Reindeer Zone, and were derived as a result of ocean plate consumption between 1.92 and 1.83 b.y. ago. The eventual collisions and associated mountain building of the Trans-Hudson Orogen were underway by 1.86 b.y. ago or earlier. By 1.83 b.y. ago, all of the oceanic plates had been consumed and the three cratons amalgamated. The resulting metamorphism ended by 1.80 b.y. ago, after which subsidence following the orogenic activity created an extensive but shallow basin for deposition of the Athabasca Group. The Precambrian Shield was intruded by minor granite at 1.76 b.y. ago and subsequent diabase at 1.27 and 1.1 b.y. ago, but has otherwise formed a stable basement for the deposition of younger Phanerozoic sedimentary rocks. (See also geology)