SUBSIDENCE-UPLIFT HISTORY AND PETROLEUM SOURCE ROCK STUDY OF CENOZOIC FORELAND BASIN OF NW HIMALAYA, H.P., INDIA

Abstract

The Sub-Himalayan Cenozoic foreland 'basin is one of the youngest features in the evolutionary history ofthis mountain ninning as foothill belt along the southernmost parts ofthe Himalaya. The Cenozoic sediments of the Himalayan foothills have been target area for oil exploration for more than a decade. Subsidence-uplift history of the basin affects the thermal maturation of organic matter to generate petroleum. The Cenozoic sediments occurring to the south of Main Boundary Thrust (MBT) were subjected to the present study. These sediments belong to the Early Cenozoic (Subathu, Dagshai, and Kasauli formations) and the Late Cenozoic Lower Siwalik Formation. The Subathu Formation (Paleocene-Late Eocene) forms 1500 m thick sequence of sedimentary rocks dominated by shale, siltstone and limestone with minor intraformational conglomerate, quartz arenite, and arenites. Shale/siltstone sequence of the Subathu Formation breaks into lead pencil-type fragments. The Subathu sediments are deposited in a shallow marine environment at around 60 to 40 Ma. The contact between the Subathu and pre-Cenozoic rocks is a disconformity. The transition between the Subathu and the Dagshai formations may seem to be abrupt. Subathu is overlain by the Dagshai Formation (Late Eocene - Early Oligocene) which consists of 600 mthick sequence of gray and purple claystone/shale alternating with the fine grained, greenish, hard, impure sandstone. The Dagshai sediments are deposited in brackish, fresh water or distal alluvial fan and fluvial system. The deposition time of the Dagshai is -30 Ma to 24 Ma. The overlying Kasauli Formation (Middle Oligocene - Early Miocene) is marked by 2100 m thick sediments characterized by massive pale gray to buff coloured micaceous sandstone alternating with the purple and gray shale. The lower contact of the Kasauli with the Dagshai is transitional, but the upper contact with the Lower Siwalik is a thrust. The Kasauli sediments are deposited in alluvial fan or fresh water at 23.8 Ma. The Kasauli Formation is overlain by the Lower Siwalik (Middle Miocene) Formation comprising 1700 mthick sequence ofalternating sandstone-shale/ siltstone with minor amount ofconglomerate. The Lower Siwalik Formation was deposited in alluvial plains in dry and wet seasons. Its depositional age is ca. 15 II to 10 Ma. The paleocurrent patterns of the pre-Siwalik and the Lower Siwalik rocks are from north to south. The Main Boundary Thrust has played important role in juxtapositioning of different older stratigraphic horizons and tectonic units with the Siwalik Group. Its activity started in Middle Miocene time (15-10 Ma) and reached its peak activity around 4-5 Ma. Sandstone and shale samples were collected from stratigraphic sections of the Subathu, Dagshai, Kasauli and Lower Siwalik formations along different traverses in areas of Nahan- Sarahan, Kalka-Kasauli-Dharampur, Dharampur-Subathu, Dharampur-Kumarhatti and Kumarharti-Solan of the foothills of the Himachal Pradesh (H.P.). The samples thus collected, were analysed for petrography, illite crystallinity, fission track analysis, total organic carbon (T.O.C.), vitrinite reflectance (VRo), rock eval analysis and hydrogen indices etc. with a view to decipher the subsidence-uplift history and the source rock evaluation of these Cenozoic sediments for petroleum. The modal analysis of sandstone samples when plotted on QFL are classified as e quartzite-sublitharnite, sublitharenite-litharenite from the Subathu to Dagshai respectively, and litharenite for the Kasauli and Lower Siwalik formations. However, considering framework constituents and matrix classification (Pettijohn, 1975), sandstone samples may be classified as: quartz arenite, sublitharenite (Subathu), lithic graywacke (Dagshai), lithic graywacke and litharenite (Kasauli and Lower Siwalik formations). The petrographic studies also indicate that the sandstones of the various formations exhibit a vertical upward discernable pattern in these compositions. These upward patterns are (a) decrease in the amount of monocrystalline quartz, (b) increase though small of detrital feldspar and polycrystalline quartz %, (c) increase of lithic constituents (metamorphic, igneous and sedimentary), and (d) increase in higher grade metamorphic rock fragments. The QFL plot also suggests recycled orogenic provenance for these sediments. The paleocurrent patterns of pre-Siwalik and the Lower Siwalik rocks are mainly from north to south. In the early stage of continental collision, the detritus was largely derived from sedimentary and very low grade metamorphic rocks from the proto-Himalaya supracrustal Indian margin rocks which are mainly Late Proterozoic to Cambrian pelitic rocks. Towards the late stages ofCenozoic deposition, the diversity of source rocks is observed. Ill Lack of wackes in Subathu Formation indicates relatively quiet tectonic conditions with very low rate of erosion and uplift. After Subathu time, in younger rocks, continual increase in lithic fragments inthe sandstones suggests relatively higher uplift rate. The X-ray studies indicate that the dominant clay minerals in shales and matrix of sandstones are illite, kaolinite, mixed layer, chlorite, vermiculite andmontmorillonite in order of decreasing abundance. The crystallinity of illite decreases with younging of sediments. The relation of sharpness ratio and crystallinity index (C.I) in pre-Siwalik and Lower Siwalik sediments is uniform and linear. The mean 20 values ofC.I. showed that the Subathu (0.37) and Dagshai (0.42) formations are both at the same stage of "late diagenesis", the Kasauli Formation (0.44) is at the lower stage ofmiddle diagenesis", and the Lower Siwalik Formation (0.53) is at the middle stage ofdiagenesis. The most dominant clay minerals in the shale and matrix ofsandsone samples are illite, kaolinite, mixed-layer, chlorite, vermiculite and montmorillonite in decreasing order of abundance. The illite peaks are sharp and well defined. The percentage ofillite varies from 50 to 82 in Subathu, 73 to 99 in Dagshai, 77 to 89 in Kasauli and 67 to 70 in the Lower Siwalik shales. The percentage ofkaolinite varies from 14 to 28% in Subathu, 8to 26% in Dagshai, 6to 19% in Kasauli and 24 to 28% in the Lower Siwalik shale samples. The percentage of chlorite in shales ranges from 20 to 30% in Subathu, 1 to 7% in Dagshai, 4 to 6% in Kasauli formations. The montmorillonite percentage is very less (<9.5) and is found only in the younger sediments. The clay matrix of sandstone samples shows variations similar to those found in shale samples. The mean illite intensity ratio (0.7 to 0.4) from Subathu to Lower Siwalik formations generally shows little recrystallization ofillite. The zircon Fission Tracks (FT) in all Cenozoic sediments have very wide age range with Gaussian peak width "w" more than 18 and thus suggest that (a) the FT in zircon grains are unreset, implying thereby that the FT in the zircon grains could not have been annealed and FT ages reveal exhumation ages ofthe provenance, and (b) the zircon grains were derived from multi FT sources. The FT ages of zircon grains range from 557 to 42 Ma for Subathu; 515-16 Ma for Dagshai; 529-12Ma for Kasauli, and 363-9 Ma for Lower Siwalik sediments.

The Cenozoic sediments have a wide distribution in zircon FT ages, and the distribution of peak widths (W>19) is higher than that of reference sample (Fish Canyon, W=16). Thus, fission tracks in zircon grains ofall samples are unreset. However, the FT in apatite grains are reset. The x2-ages from clubbed samples of the Subathu, Dagshai, Kasauli, and the Lower Siwalik formations are 58.8, 26.2, 22.2-14.2 and 9.7 Ma respectively. The lag time between %2- ages and independently-determined depositional age is less than 5Ma between cooling age ofthe youngest zircons and depositional time. The detected Gaussian peaks in all these formations show shifting towards younger ages indicating more and more contributions from younger sources of sediments. The source area for the zircon grains, based on paleocurrent directions, appears to be north and northeastern Himalayan terrains. The distribution of zircon FT dates indicates Paleozoic and Mesozoic sediments/igneous rocks as source of sediments to the Himalayan foreland basin. The first indicator of Cenozoic Himalayan orogeny is identified in the Middle Sub'athu around 40.7 Ma. Based on fission tracks (FT) in zircon grains the upper limit of depositional ages as estimated by youngest Gaussian peak age of individual formation are: Subathu ca. 39.1 Ma, Dagshai ca. 25.1 Ma, Kasauli ca. 25.2-15.2 Ma, and Lower Siwalik ca. 14.6 Ma. These ages are consistent with the ages determined by biostratigraphy (Arya, 1998), magnetostratigraphy (Sangode, 1997) and argon-argon methods (Najman, 1997; Harrison, 1992). On applying the Gaussian Best Fit method onclubbed (bulk) samples of each formation, it is found that the percentage of grains of old FT ages continually decreases upward and the younger FT ages continually increases upward from the oldest Subathu Formation to Lower Siwalik Formation. This indicates continual rising up of the source (Himalaya) which contributed sediments to this Cenozoic foreland basin. The oldest age peaks are mostly characterized as static type in all the formations. The possible source area for the static peaks is unreset and / or reset terrains located on the northern side of the Cenozoic belt with the low rate of uplift. The absence of regular pattern in the age distribution of relatively younger age peaks may be related to the recycling of sediments which is also corroborated by petrographic evidences in the form of sedimentary lithics, abraded quartz overgrowth and rounded zircon, tourmaline and garnet found in these sediments. These two types ofpeaks and "wide distribution" ofFT ages in zircon grains in various formations ofCenozoic sediments show that the uplift rate in the provenance was not uniform. Petroleum hydrocarbons are generated by the thermal degradation of sedimentary organic matter in the temperature limit of about 60°-180° C. The peak oil generation takes place at temperature ofthe order of 120° C. It is important to note the preserved fission tracks in apatite grains indicate that these sediments have attained temperatures required for generation of petroleum hydrocarbon In view of this, petroleum source rock studies were conducted to estimate abundance of organic matter in terms of Total Organic Carbon (TOC) content, quality of organic matter in terms of type of kerogen and the thermal maturation of organic matter to generate the hydrocarbons. The TOC in shales varies from 0.09% to 0.3% (mean 0.2%) in the Subathu Formation, 0.07% to 0.37% (mean 0.15%) in the Dagshai Formation, 0.09% to 0.6% (mean 0.2%) in the Kasauli Formation and 0.06% to 0.17% (mean 0.1%) in the Lower Siwalik Formation. The kerogen is mainly of type with major terrestrial component. This type of kerogen, which is prone to generate mainly gas but little oil, is found in all formations. The mean values of vitrinite reflectance (VR.) are 1.2% for Subathu, 0.9% for Dagshai, 0.7% for Kasauli and 0.8% for the Lower Siwalik formations, indicating thereby that organic matter is matured to various levels to generate petroleum hydrocarbons. Based on fission track analysis, illite crystallinity indices and vitrinite reflectance the maximum paleotemperature of various Cenozoic formations are estimated to be 100-120°C (110°C) for Lower Siwalik, 120-130°C (125°C) for Kasauli, 130-140°C (135°C) for Dagshai and 140-160°C (150°C) for Subathu formations. Analysis offission tracks on zircon using External Detector Method (EDM) indicates that sediments of all the formations of the Cenozoic foreland basin have not subsided to the depth level corresponding to the closure temperature of fission tracks in zircon (240±25°C). The basin subsided maximally to the depth level corresponding to temperatures ofthe order of 160° C. The fission tracks preserved in the apatite grains in the sandstone of various formations appear to be

reset. This resetting is also indicated by the ages of FT in apatite, which are younger to their depositional ages (8 Ma inDagshai, 13 Ma in Kasauli and 10 Ma inLower Siwalik Formations). Spatially and temporally, the rate of subsidence in all parts of the foreland Cenozoic basin has not been uniform. Subsidence history shows breaks (40-30 Ma and 6.5-5 Ma) in the continuous trend in the region of Surinsar, Jammu hills. The present data indicate that the Kasauli sediments exhumed 1 km before 13 Ma. The Dagshai sediments were exhumed at least 0.5 km since 13 to 8 Ma, i.e. 0.1 km/Ma. The exhumation of sediments in the foreland basin is estimated to be about 2.5 to 3.0 km in the present area, i.e. 0.3-0.4 km/Ma since 8 Ma to present time. Thus, the total exhumation of the sediments is around 4.0-4.5 km since the beginning of exhumation. Thus, the rate of exhumation of foreland sediments from early to end of Middle Miocene (from 13-8 Ma) is very low (0.1 km/Ma) but from the Upper Miocene to present is high (-0.3-0.4 km/Ma). The history of the northern margin of the foreland basin may be different from the southern part. The northern margin of foreland basin between Nahan Thrust and MBT started uplifting, due to MBT activation >13 Ma ago according to apatite FT results, and contributed detritus for the younger sediments as additional source. The subsidence curve of the Subathu sediments indicates that they are within petroleum "window zone" stage of maturation. The petroleum hydrocarbon generation started at -25 Ma (Middle Miocene) when the Subathu reached a burial depth of -2500 m and continued till to about 13 Ma back when these sediments reached a depth -4500 m, i.e. the time of inception of uplift. For all formations, the hydrogen indices (HI) are below -150 mg C02/gTOC, indicating low prospects of liquid hydrocarbon. The Subathu shales with the VRo%~1.2 and Tmax>470oC indicates late stage of metagenesis; the Dagshai Formation with the VR.,%- 1 and Tmax of 450° C and the Kasauli (Murree) sediments with the VR„%~ 0.9 and T^ between 450°-430°C indicate initial to middle stage (catagenetic) of maturation of organic matter; and the Lower Siwalik Formation with the VR.,%- 0.8 and Tmax 445° C indicates that it is in the gas stage catagenetic maturation to generate petroleum hydrocarbons. These results indicate that the shales ofthe Cenozoic sediments are matured enough to generate petroleum hydrocarbons. The kerogen type-Ill indicates that the principal organic matter is prone to gas generation. The source rock of

the Cenozoic sediments is, thus, matured to generate mainly gas. These results are supported by the gas seepage and live oil shows in the Siwalik foreland basin. However, since the TOC is generally less than 0.5%, the chance of generation of commercial quantities of petroleum hydrocarbons is less, and it is important to note in this regard that the exploration efforts of Oil and Natural Corporation during the past have not met with success as yet.