The Zanjan Quadrangle covers part of the western Alborz Mountains and a number of parallel ranges and depressions in the northwestern extension of central Iran.
The oldest formations exposed are low-grade metamorphic rocks, the non-metamorphic shales of the Kahar Formation, and granites (Doran Granite), forming together a basement complex of Precambrian age. This complex is overlain in the Soltanieh Mountains by about 2000 m of sandy, shaly, and dolomitic rocks, the Infracambrian Group, which is divided into the Bayandor, Soltanieh, Barut, and Zaigun Formations. The Zaigun passes into the Early Cambrian Lalun Sandstone (600m), and this into the richly fossiliferous Middle Cambrian to Ordovician Mila Formation (500 m) composed mainly of dolomite, limestone, and shale. No Silurian and Lower Devonian rocks are known in the Zanjan Quadrangle, and Upper Devonian and Lower Carboniferous limestones are limited to a few outcrops in the Talesh Mountains. Early Permian sandstones and shales (Dorud Formation, 100 m) and the Late Permian Ruteh Limestone (200 m) have a wider distribution and overlap various older rocks with a distinct disconformity. Triassic rocks are missing, probably as a consequence of intensive pie-Jurassic erosion. Plant-bearing sandstones, shales and conglomerates of the Rhaeto-Liassic Shemshak Formation (1000-2000 m), containing minor coal deposits, rest on an erosional surface truncating various Paleozoic and Precambrian formations. The Shemshak is followed conformably by the Middle-Late Jurassic Lar Limestone (500 m). Cretaceous shales and limestones are the oldest rocks exposed in the south-western part of the quadrangle, where they are in places slightly metamorphosed; in the north¬east they are replaced by a purely carbonatic sequence in places more than 1000 m thick. The Eocene is represented mainly by thick volcanic rocks and tuffaceous sediments (Karaj Formation, 3000-4000 m), locally with thin basal limestones (Ziarat Formation) and conglomerates (Fajan Formation) resting with pronounced unconformity on various older formations. Granites and granodiorites of Late Eocene or Oligocene age form several large intrusive bodies in the Eocene and older formations. The Oligocene-Early Miocene Qom Formation (more than 600 m) is the youngest marine deposit and is limited to the southwestern part of the quadrangle; it is overlain by the Miocene Upper Red Formation (more than 2000 m), and similar Neogene red beds fill the Manjil depression in the northeast. The youngest, unfolded deposits are Plio-Pleistocene red beds, conglomerates, freshwater sediments, and Quaternary river terraces, loess, and extensive gravel sheets.