Text|Geography
Editor|Geography
1. Introduction
In sedimentary basins, stratigraphic columns are a key indicator for interpreting their geological history; But often stratigraphic columns may not clearly infer the tectonic and stratigraphic control that the basin underwent during its existence.
This is because of the compaction effect that occurs during burial; The internal structure of each rock cell changes over time. Therefore, decompressing compacted sedimentary stratigraphic columns (restoring the original thickness of stratigraphic units) is critical to understanding and modeling the deposition of the sedimentation system during the deposition process and revealing the internal structure of the basin throughout the sedimentary history.
The recovered thickness plays an important role in understanding the settlement of the basin and reconstructing its evolutionary context over geological time. The area around the fold and thrust zone in the northeast region of northeast India includes a northeast-southwest basin known as the Assam Basin.
It resembles a foreland formation and has attracted considerable attention in the Lower Assam Basin due to the right-handed slip movement of its Lower Brahmaputra (Kopili) fault, as well as the hydrocarbon potential of the Upper Assam Basin.
Studies that have previously described the tectonic background and sedimentary environment of the Assam Basin's foreland basin have relied primarily on magnetic, gravitational, 2D/3D reflected seismic and outcrop studies. These studies have limitations in understanding the subsurface structural systems that control basin configurations. However, there is still a lack of detailed documentation to elucidate the settlement history of the basin through quantitative analysis.
Although many oil and gas exploration companies, such as Oil and Gas Corporation (ONGC) and Oil India Limited, have already explored in the Upper Assam Basin, the results have not been made public due to proprietary issues.
This case study is a pilot study conducted in a natural laboratory in the Upper Assam foreland basin, drawing on drill data recently acquired by the National Commander-in-Chief (DGH) of India to elucidate the tectonic controls the basin undergoes during its life cycle.
Tectonic settlement analysis was performed by performing destratification analysis calculations on ten boreholes in the study area, which aimed to (a) interpret the settlement history of the basin; (b) to explore how tectonics and settlement control the evolution of basins; and (c) the construction of an evolutionary model to elucidate the growth and evolution of the Upper Assam foreland basin throughout the Cenozoic period.
2. Geological background of the Upper Assam foreland basin
- 2.1. Geodynamic framework of the basin
The Upper Assam Basin is a composite foreland basin located between the foothills of the Northeast Himalayas and the Assam-Arakan thrust zone. Structurally, the basin is located in a regional triangle between the Himalayan Frontal Thrust (HFT) and the Naga Thrust (NT).
The tectonic configuration of the basin is strongly influenced by tectonic interactions between Himalayan orogeny in the north, the Mishmi thrust fault in the east, and the Assam-Arakan Mountains in the south.
It is characterized by an asymmetrical slope that slopes slightly to the Himalayan foothills in the north and the Naga Mountains in the south. The northeast side of the basin is terminated by the Mishmi thrust fault and partially disturbed by the uplift of the Shillong plateau crust in the southwestern part.
The convergence of the Indian plate with the Eurasian plate and the collision with the Myanmar plate led to strong bending and tectonic deformation of the central basement of the basin, forming several highlands and low-lying areas.
The basal structure and overlying sedimentary coverings appear to be plateaulic, showing lightly elongated folds cut by NE-SW or ENE-WSW trending fault profiles. Due to continuous convergence and collision over a long geological period, these sediments have undergone intense uplifts, folds and fault movements.
- 2.2. Lithologic stratigraphic framework of the basin
The Precambrian basement of the basin is composed of crystalline metamorphic rocks, including gneisses and granites. Above the basal are Paleocene, Neoocene and Quaternary deposits, including the Langpar, Lakadong/Therria, Prang, Narpuh and Kopli formations (Paleocene to Eocene), Barail (Oligocene), Tipam and Girujan (Miocene), Namsang (Miocene), Dhekiajuli (Pliocene), and alluvial cover (Pleistocene to the present).
These sediments form in a variety of shallow sea, delta, and fluvial environments, including limestone, shale-sandstone, coal-shale interactions, heavy sandstone, variegated and multi-colored clays, interbedded sandstones, and thick alluvium.
In the current period, the basin receives sediments from the north, south and east directions due to being surrounded by huge folds and thrust zones.
3. Data and Methods
The study area covers an area on the shelf of the Upper Assam foreland basin, 80 km long and 40 km wide, and includes stratigraphic details of ten boreholes (DBR-1, NMD-1, BP-5, BP-6, BP-8, BP-15, SC-1, SC-4, NK-496 and NK-511) within the study area.
These holes penetrated the Tier III and Tier IV sedimentary columns of the basin, including the basement. Settlement analysis was performed using the stratigraphic details of these ten boreholes.
Since current sedimentary columns do not directly reveal the amount of sediment accumulated in past geological periods, a decompression technique is used. The original thickness of each sedimentary layer is restored by assuming an exponential relationship between porosity and depth, according to which porosity decreases exponentially with depth.
Paleowater depth information can be determined by different sedimentary markers, mainly benthic microfossils, plant and animal clusters, sedimentary facies, and unique geochemical signatures.
The determination of paleowater depth is usually based on sedimentary systems and paleontological studies compiled from borehole data and seismic sequence studies of the Upper Assam Shelf, where Tertiary and Quaternary strata are decompressed and dissected in the opposite direction.
Palaeocene strata were allocated paleowater depths of 0-100 meters; Oligocene strata are 0–10 m; Miocene strata 0–80 m; The Upper Miocene strata are 0–5 m. Sea level change has a small effect on dorsal stripping.
Recording short-term sea level fluctuations introduces uncertainty about sedimentation models. Therefore, with this in mind, sea level change in the calculation of the subsidence model in the present study is not taken into account.
The lithological composition of each geological formation is primarily derived from completion reports (WCR) and logging data for each borehole, as well as published literature, and tectonic settlement curves are generated for each hole in the study area. Through different contour maps, they are visualized in order to understand the tectonic control and evolution of the basin over different geological periods.
4. Results and interpretation
- 4.1. Basin structure and underground strata
The Upper Assam foreland basin has a Precambrian basement whose geometry is tectonically controlled by multiple anticlines and faults. This is evident from the regional geological profile of the study area, where the depth of the basement varies between 3780 and 4430 m.
In the eastern part of the study area, the basal is thicker and thinner. The basement is covered with sediments from the Tertiary and Quaternary periods. The profiles of the DBR-1, NMD-1 and SC-1 holes clearly show that the Tertiary and Quaternary sediments in the study area show changes in underground lithology from west to east.
These sediments appear to be thicker in the eastern part, and during the Miocene, sediment loads created accumulation spaces on a local/regional scale along with the activity of tectonic faults.
Settlement was accompanied by Miocene tectonic fault activity, resulting in multiple pull-together fault structures that became appropriate basins for the accumulation of Cenozoic sediments within the basin.
- 4.2. Sedimentation analysis
Tectonic settlement in the southwest of the study area (DBR-1 and NMD-1 holes) remained slow during the Paleocene period, ranging from 28.9 m to 219.5 m. From the Oligocene to the Miocene to the Pliocene-Pleistocene, the subsidence gradually increased, increasing in a step-like manner.
In the middle of the study area (holes BP-5, 6, 8 and 15), Paleocene settlement remained slow. Conversely, from the Oligocene to the Miocene, the settlement gradually increased, and from the Pliocene to the present day experienced a step-like increase.
During the Miocene period, subsidence in the central region remained significant. In the northeast of the study area (holes SC-1, 4 and NK-496, 511), settlement showed a similar trend during the Paleocene, Neoocene and Quaternary, and the tectonic settlement remained significant in the Oligocene, Miocene, and Pliocene-Pleistocene periods in the study area.
The sedimentation curve shows an initial phase of slow settlement in which the sedimentation rate remained low (i.e., during the Paleocene). Since then, these curves have gradually increased, showing a concave upward trend. Further increases in subsidence between the late Miocene and the Pliocene-Pleistocene indicate that subsidence is still accelerating.
During the Paleocene (66-56 million years ago), tectonic settlement varied between 15.0 m and 130.0 m, with records showing the largest in the northeast and the smallest in the central and southwestern regions.
A gradual increase in sedimentation was observed during the Eocene, with tectonic deposition varying between 110.0 m and 260.0 m in the Early Eocene (56-47 million years ago), between 160.0 m and 400.0 m in the Middle Eocene (47–42 million years ago and 42–38 million years ago), and between 480.0 m and 750.0 m in the Late Eocene (38–33 million years ago).
Throughout the Eocene period, subsidence was greatest in the northeast and smallest in the central and southwestern parts of the study area. During the Oligocene, subsidence increased steadily, ranging from 820.0 m to 1240.0 m.
Settlement continued to be significant during the Miocene, with tectonic sedimentation varying between 1020.0 m and 1420.0 m in the Early Miocene, 1100.0 m to 1900.0 m during the Miocene, and 1100.0 m to 1950.0 m in the late Miocene.
During the Miocene, the northeast continued to experience greater subsidence relative to the central and southwestern parts of the study area. During the Pliocene-Pleistocene (5.2-0.012 million years ago), accelerated subsidence occurred and remained stable. During this period, subsidence varied between 1770.0 m and 2000.0 m, with the southwestern part of the study area exceeding that of the central and northeastern regions.
5. Discussion
- 5.1. History of subsidence in the Upper Assam foreland basin
Tectonic interactions between the Himalayan orogenic belt and the north, the eastern part of the Mishmi thrust fault, and the southern part of the Assam-Arakan Mountains have influenced the structural setup of the basin at different geological times, with the onset of tectonic settlement recorded about 66-56 million years ago.
During the period about 66–56 million years ago and about 56–33 million years ago, the sedimentation rate was relatively low (average 5.16–13.52 m/ma). It gradually accelerated during the Oligocene (about 33–23 million years ago) and Miocene (about 23–5.3 million years ago).
At this stage, the basin slopes southward and southeast due to the subduction of the Indian plate under the Myanmar plate. As a result, sediments flow from north to south and northeast to southeast. This resulted in a significant change in the settlement rate of the basin.
During the Pliocene-Pleistocene, the basin gradually sloped to the north and northeast during the Pliocene-Pleistocene, and in the time thereafter, the south, east, and north were closed.
During the Pliocene-Pleistocene, settlement remained accelerated as the basin continued to supply sediments to the southwest to fill reservoir spaces. In addition, the extrusion forces from these three directions and the relative motion of the plates structurally controlled the structural configuration of the basin throughout the Cenozoic period.
The settlement curves obtained from the studied boreholes show that within the basin, settlement remained slow during the Paleocene period, while it gradually accelerated from the Oligocene to the Miocene to the Pliocene-Pleistocene. These curves have a concave upward (or concave downward) tendency and reflect the foreland structure of the Upper Assam Basin.
These contours depict the gradual acceleration of settlement as tectonic loads migrate landland and the basin curves in its curved shape. Due to the migration of tectonic loads and continued sedimentation, the Assam Basin expanded over time and formed foreland structures.
In the proximal section, the basin descends immediately due to this increased tectonic load, and those that can move away from the central part tend to react later. From the analysis of the tectonic settlement curve, the maximum amplitude of tectonic settlement is observed to be about 2 km, which is within the maximum limit of the foreland basin on a global scale.
The history of subsidence in the basin remained rich during the Oligocene, Miocene, and Pliocene-Pleistocene-Pleistocene. Tectonic settlement during the Oligocene remained accelerated and averaged about 548–983 m with a standard error of 39 m.
This period was accompanied by severe tectonic movements, including the subduction of the Indo-Burma plate and the southward dip of the basin. The Miocene period remained significant because of the extensive sediment of river sediments. These deposits are deposited in marine intrusion environments east of the Himalayan front.
Sedimentary housing spaces for Miocene sediments were created primarily due to tectonic loading and settlement. Therefore, tectonic settlement during the Miocene period continued to remain significant, averaging about 983-1437 meters with a standard error of 63 meters.
Subsidence continued during the Pliocene-Pleistocene period, averaging about 1437–1841 m with a standard error of 55 m. At this stage, the basin slopes to the north, and tectonic movement occurs in the south, with tectonic movement occurring in the pushsheets. Therefore, as a result of this tectonic activity, many deformed structures formed and gradually became traps for sediments.
The rate of tectonic deposition also continued to be significant during the Oligocene-Miocene to Miocene-Pleistocene-Pleistocene periods. The average rate of Oligocene is about 70-30 m/ma, and the standard error is 4 m/ma. The average rate in the Miocene is about 30-10 m/ma, and the standard error is 6 m/ma; The average rate during the Pliocene-Pleistocene period is about 10-70 m/ma, with a standard error of 10 m/ma.
Geological parameters have a significant impact on the reconstruction of tectonic settlement, including sediment compaction, lithology, and changes in paleobathymetry. However, in the impact of tectonic settlement, sea level rise and fall play a lesser role, and lithological details usually play a dominant role, as the porosity of the sediment decreases as the thickness of the covering the formation increases during sedimentation.
In this study, the thickness of each geological layer and its lithological details were considered based on the stratigraphic records of the study boreholes and different published literature. The tectonic settlement curves produced by each study borehole were corrected for paleoseafloor depths.
The DBR-1 borehole was used as an example to show the tectonic settlement difference before and after the correction, avoiding that this correction would produce a paleoseabed depth error range of about 111.0 m during the Pliocene-Pleistocene period. This period is the most important for sediment filling and tectonic movement within the basin.
Corrected tectonic settlement presents a realistic graphical representation of the tectonic stages that the basin undergoes over geological time, and this correction method can improve the accuracy of analyzing the settlement history of sedimentary basins.
The evolution of the Upper Assam foreland basin is controlled by tectonic interactions of ancient and newer fault systems and is stratigraphically regulated by sedimentary processes over different times, and the conceptual map highlights the evolutionary stages of the Upper Assam foreland basin.
The early stages of basin evolution were controlled by block fault activity, dividing the underground into different blocks. Indo-Burma plate interactions during the Ordovician activated tectonic movements of faults within the basin.
During this period, a regressive environment persisted, and the basin experienced several tectonic uplifts. Subsequently, the sedimentation rate of Miocene and post-Miocene sedimentation was rapid. Much of the basin is under fluvial sedimentary environments. From the late Miocene to the morning Miocene, the ocean retreated to the south and southwest.
During the Pliocene to Pleistocene, the basin was covered with thick alluvium. Today, the basin is part of a sloping continental shelf southeast of Assam, surrounded by a huge fold-thrust fault zone. Recent basin modelling studies, combined with boreholes and multiple geoscience data, have updated regional geological models of the Upper Assam foreland basin, including its evolutionary history.
These studies further suggest that the basin experienced a southward dip during the Ordovician-Pliocene period, and then a northerly dip during the Pliocene-Pleistocene-Pleistocene.
As a result of this tectonic activity, as well as the presence of fold-thrust fault zones around the basin, sediment inputs are now received from three directions (north, south and east). The tectonic tilt of the basin, sediment loading from the boundary fold-thrust fault zone, and basin settlement over geological time have combined to influence the tectonic configuration of the Upper Assam foreland basin.
6. Conclusion
Based on the detailed analysis of the underground strata of the boreholes in the study area, it is proved that the Upper Assam foreland basin has undergone strong subsidence in geological time, and the tectonic subsidence of the upper Assam foreland basin is divided into four different stages.
During the Paleocene period, the basin experienced a slow sinking. It gradually increased during the Oligocene and reached a rapid rate during the Miocene, and during the post-Miocene sedimentation (Pliocene-Pleistocene), the tectonic subsidence within the basin remained accelerated.
The sinking curve obtained from the studied boreholes has a convex upward pattern, indicating that the basin has gradually developed a foreland configuration over time and is now a southeast-sloping shelf surrounded by relative thrust fold-thrust fault zones. During the Ordovician-Miocene, tectonic subsidence continued to be significant.
During the Ordovician, the average drop to about 548-983 m with a standard error of 39 m; During the Miocene, the average sink was about 983-1437 m with a standard error of 63 m.
During the entire Pliocene-Pleistocene, the average sinking was about 1437-1841 m with a standard error of 55 m, and the entire basin experienced a tectonic subsidence of about 2 km during its evolution, with an average sinking rate of about 30 m/ma.
bibliography
- De S. Barber, D.F. Stockley, B.K. Horton, Rhode Island, Koshnau, Cenozoic excavations and foreland basin evolution in the Zagros orogenic belt during the Arab-Eurasian collision in western Iran, Tectonics, 37 (12) (2018), pp. 4396-4420
- G.C. Bond, Mass. Komintz, Construction of early Paleozoic microclinic tectonic settlement curves in the Rocky Mountains of southern Canada: effects on sedimentation mechanism, fracture age and crustal thinning, Acta Geologica Sinica, 95 (2) (1984), pp. 155-173
- S. Borgohein, J. Das, aka Saraf S. Singh, S.S. Barral, Structural control of topography and river morphological dynamics in the Upper Assam Valley, India, Jordin. Journal of China, 29 (1) (2017), pp. 62-69