The relief in the administrative territory is extremely uneven, specific to the mountain area with gorges on the course of the two Jiu (Eastern and Western Jiu). Mountain ranges bordering the basin are: Vâlcan to the South, Retezat to the North, Parâng and Godeanu to the East to West, the maximum altitude of 2,519 m being in Parângul Mare Peak. Along the Eastern Jiu a meadow and terrace were formed, the latter having a smooth surface and being a little fragmented. Because it is an intermediate step between the foothills and the meadow, it was valued by the agricultural crops and its presence along the main communication routes.
The Eastern Jiu and four of its tributaries: Maleia, Staicului, Slatinioara and Salatruc flows through Petrosani. The effect of the abundant precipitation is that there are a lot of groundwater and springs in Petrosani.
The city lying among Parang, Godeanu, Retezat and Vâlcan Mountains meets well-defined characteristics of a hydrographic basin, and of a morphological and geographical depression generally.
Considering the shape and position it holds in the whole relief, Petrosani is part of that huge longitudinal corridor that divides the Meridional Carpathians. That is a depression corridor with strong regional accents given not only by the late modeling of Meridional Carpathians, but also by their own structure and by the tectonic defining of that unit in various geological stages.
The first outline of the basin where the municipality of Petrosani lies is from early middle cretaceous orogenic stage of the Carpathian overthrust, but only several million years later in the Oligocene when sinking and sea invasion produced, it started the sedimentation cycle in that basin .
The oldest deposits filling the basin are made of red clay cement conglomerate. Fragments of crystalline rocks and their arrangement in a torrential stratification are a valuable sign for the whole region landform of those times. The surrounding mountains have formed in Oligocene sea a real archipelago undergone to an active erosion. Short fast waters cutting the islands of the archipelago have transported the coarse silt, which filed the seabed and became the conglomerates got up to the surface by the erosion on the southern and western sides of the basin. Oligocene to Miocene transition meant not only sedimentation intensity slowing down and changing the nature of formations (alternating marl and clay), but also the beginning of a deposit phase of a great importance for the region.
In the new clay-marl horizon, which a thickness of approximately 300 m and deposited in different conditions, there were numerous intercalations of sandstone, carbonaceous shale and coal, which attests a quiet region speaking about the earth crust turmoil and a warm climate favorable to lush vegetation growth, which have been formed coal. In the coal horizons there were tree trunks remains of Sequoia, Walnut and Elm. Coal here was also formed from the leafage and trunks of the trees growing today only in places with warm climate, such as Cinnamon and bay trees. The deposits in the soil of Petrosani basin have continued until the end of the Tertiary. In that period there have been deposited alternating conglomerates and sandstones, or gravel and boulders with torrential stratification as well. Of all the sediments deposited in the basin of Petrosani, only the horizon from Oligocene to Miocene transition period contains coal beds.
The geologist Nicolae Oncescu considers that the formation of successive beds of coal produced in this way: “frequent alternation of marine beds made of sandstone and marl with proper lagoonal layers, represented by bituminous rocks and freshwater deposits, represented by coal, allow us to distinguish the numerous sedimentary cycles due to some Eustatic movements of the basin. In such a cycle there were formed marine deposits at the beginning, sometimes conglomeratic, then lagoonal deposits with shale deposits, the cycle ending with a freshwater phase during which a peatery settled in the basin forming a coal bed”.
Because of the rhythmic slowdown and cease of the basin’s sinking, there were formed several coal horizons, very unequal developed and uneven incarbonized. The quality of the coal from the subsoil of Petrosani is due to the tectonic conditions from last part of Tertiary. Therefore, as tectonic movements were very strong, the coal from Petrosani has superior quality. By speeding up the incarbonization process, coal qualities have gained great industrial importance (cokes and distilled). The depression, during its existence as a gulf of the tertiary sea, suffered a slow immersion, it could not be filled with sediments to become land. The silting process produced in the Pliocene and the depression became land. That land was involved in the movements of ascension in block of all of what had become Carpathian chain. After the decrease in water levels, the waters started to erode the land, which previously contributed to its forming. Once the waters started to erode the relief, they removed the layers one by one, and in their valleys they formed gorges. The existence of a succession of rocks with different action at the erosion process, allowed carving of interesting shapes (steep walls, tanks) which reinforce the picturesque region.
The soil in Petrosani Depression belongs to the automorfic and hydromorphic soils, yellowish- brown and brown sylvan podzolic.