Mineralogy and Geochemistry of the Cambrian Formations in Salt Range

Abstract

The Cambrian formations exposed in salt range were studied to understand the palaeo-environments with the help of their mineral composition, chemical composition, lithological variations in the field and the grain size variations. The Cambrian formation named as Khewra sandstone, Kussak formation, Jutana Dolomite and the Baghanwala formation were investigated in field to know their relations with the overlying and the underlying formations, detailed field lithological variations and further sub-division into units and sub-units. The reconnaissance survey of about 300 kms was conducted to select the best geological sections for the stratigraphical studies of the Cambrian rocks in the eastern, central and western salt range. The geological sections of the Cambrian rocks were measured at eight selected locations to understand the regional stratigraphical variations. About 1153 major beds (medium to thick bedded) and 11431 total beds (including micro-beds) were investigated in the field to understand the lithological variations of the Cambrian rocks exposed in the salt range. The Khewra Sandstone conformably overlies the salt range formation with occasional local disturbances and may be divided into three sub-units. The Kussak formation disconformably overlies the Khewra Sandstone and may be divided into four units on the basis of its lithology and beddin characters. The Jutana Dolomite conformably, overlies the Kussak formation and may be divided into two units. The Baghanwala formation conformably overlies the Jutana Dolomite and may be divided into three units. The Baghanwala formation is unconformably overlain by the Tobra formation of Permian age. The samples were collected from Cambrian formations exposed at Ludwah hills, Baghanwala village, Khewra Gorge, Nilawahan Gorge, Kalarwahan and the Khan Zaman Nala. The megascopic studies of about 525 samples, the sedimentological studies of 193 samples, the petrographic studies of about 202 samples, the chemical analyses of about 67 samples and the X-ray diffraction studies (clay minerals and other minerals) of about 201 selected samples were conducted to understand the palaeo-environments and any economical exploitation. The grain size analysis of the Khewra Sandstone consists of mostly silt size fraction in all the sections and may be called as the Khewra Sandstone is a chemically mature sandstone. The Sandstone was deposited in deltaic environments, representing the topset, the forest and the bottom set deltaic sequences, represented as units C, B, and A, respectively. Economically, its upper and middle units are good reservoirs for water, oil and gas due to their porosity. It can also be used as building stone and a decorative stone. The upper unit sands exposed at Ludwah may also be used as glass sands. The Khewra Sandstone represents the regression of the Cambrian sea. The Kusaak formation was deposited in intertidal to supratidal and subtidal environmental conditions, representing chemically matured beach sands, supratidal lagoonal deposits, the subtidal sands and the carbonates. Economically, the oolitic sands and carbonates of the Kussak formation could be good reservoirs of water, oil and gas. The lagoonal clays/shales consist of comparatively high potassium and may be used as potassium fertilizers after enrichment. The stromatolitic beds may be used as decorative building stones. The Kussak formation represents the transgression of the Cambrian sea. The Jutana Dolomite is a secondary dolomite and was deposited in the intertidal to subtidal conditions with occasional supratidal conditions. The Jutana Dolomite shows primary and secondary porosity and may be a reservoir of water, oil and gas. The dolomite is often of high grade and may be used in steel industry and other economical uses of traditional dolomites. The Baghanwala formation was deposited mostly in supratidal conditions, representing mostly lagoonal shales, clays, silts and carbonates. The Baghanwala formation represents an argillaceous unit and seals the reservoir rocks lying below it and may be called a cap rock for the Cambrian reservoirs. The flaggy siltstones/sandstones of the Baghanwala formation may be used as the building stones. The Baghanwala formation represents the regression of the Cambrian sea. The detailed field, sedimentological, geochemical and petrographic studies reveal that the Cambrian formations were deposited in an intracratonic depositional basin, formed due to rifting.