Sedimentary Sequences of Krishna River Delta to Identify Paleo Depositional Environments

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Sedimentary Sequences of Krishna River Delta to Identify Paleo Depositional Environments

https://doi.org/10.21203/rs.3.rs-4873576/v1

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Deltas play a crucial role in the economic and environmental development of the region, thus they gain much attention from the Government, as well as scientists, and researchers for different developmental activities. In this regard, the grain size studies are important to provide clues to the sediment provenance, transport history and depositional conditions. In addition to that, Foraminifera studies provide insights into the environmental changes of the deltaic regions, and they are used for environmental reconstructions. Thus, the present work focuses on textural and micropaleontological studies to identify depositional environments. Inturu core four sedimentary sequences were identified from bottom to top viz. river channel, estuarine channel, mud flats and flood plain. Nizampatnam core, seven sedimentary sequences were identified from bottom to top viz. river channel, estuarine channel, mud flats, flood plain, estuarine channel, mud flats and estuarine channel which represents sub environments of fluvial and estuarine systems. Coarse-grained and fine-grained sediments are related to the high energy, and low-energy conditions respectively. The presence of microfossils like Asterorotalia trispinosa, Asterorotalia subspionsa, Asterorotalia dentata, Asterorotalia gaimardi, ammonia tepida, nonionella and cibisides indicates an estuarine environment. Repetition or overlapping of environments indicates lateral shifting of the river course whereas, neotectonic activity or sedimentary basin subsidence or progradation of delta causes vertical shifting. The polymodal nature of river sediments indicates that these sediments might be contributed by various sources i.e. drainage basin, reworked sediments of the river bed load, river bank eroded sediments and addition of fine-grained material due to alteration of pre-existing minerals of the main sedimentary unit. The unimodal sediments suggested that the sediments were contributed by a single source or they had not undergone any post-depositional changes.

Delta

Textural

Microfossils

Paleoenvironment

Neotectonic activity

Polymodal

The deltas on the east coast of India, exhibit a variety of landforms, such as ancient channels, natural levees, ancient beach ridges, mangrove swamps, lagoons and barrier spits (Nagewsara Rao and Sadakata 1993). Deltas are the recipient of an enormous quantity of recent sediments and also provide a restricted anoxic environment, favourable for growth and preservation of vegetation and organic matter needed and suitable for the potential hydrocarbon generation and accumulation (Huang et al., 2013; Jin et al., 2020; Jamaluddin et al., 2023; Ma and Jinlai, 2023). Understanding these typical environments especially the ancient channels and beach ridges helps in demarcating potential oil resources in the exploration of paleo deltas. Sediment characteristics of riverine sediments depend on the source area, morphology of the river basin and hydrodynamics of the fluvial system (Parker, 2008; Buffington, 2012; Hassan and Zimmerman, 2012; Kulkarni et al., 2015; Blom et al., 2017; Venditti et al., 2017; Chitkara et al., 2022; Rahman et al., 2022; Nichols et al., 2023). The influence of the seasonal hydrodynamic conditions would be reflected in the textural variations of sediments. The various studies conducted on sediment physical characteristics of various environments of Krishna River but at the surface level (Krishna Rao et al.1988; Sreenivasa Rao et al.1990; Krishna Rao et al. 1990; Gandhi et al. 2000; Krishna Rao and Swamy 1991 and Nageswara Rao et al. 2005).

In addtion to that, foraminifera studies for identification of sedimentary environment and recognition of past sea level fluctuations as indicators of both natural and anthropogenic environmental stress due to their high diversity, abundance, and preservation potential (Dabbous and Scott, 2012; Carnoma et al., 2009; Buatois et al., 2010, Gandini et al., 2014). Foraminifera is sensitive to environmental changes and, it is commonly used for implications in paleoenvironmental reconstructions and sea level changes (Murray, 2006; Liu et al., 2013; Kundsen et al., 2012). Thus, the main objective of the work is to reconstruct the evolutionary history of Krishna River delta includes identification and delineate different sub-environments of Krishna River Delta and characterization of each sub-environment through textural and micropaleontological characteristics.

The study area covers the western part of a lower delta of Krishna River adjacent to Nizampatnam Bay. The cored holes fall in the area bounded between East longitudes 80⁰ 37' 40'' and 80^o 40' 16'' and North latitudes 15⁰ 54' 04'' and 16⁰ 03'^‌ 50' ' (Fig. 1). The geological formations in the Krishna River basin mainly Deccan Traps (50%), unclassified crystalline rocks of Archaeans and Precamberain sediments of Cuddapah. Vindhyan and Kurnool super groups and khondalites of the Estern Ghats (Fig. 2) and exhibit uniformity in the mineralogical charter and chemical composition. The normal compact dolertic to basaltic (Thoeliite) type occurs over large areas in the upper basin. The unclassified crystallines are comprised of the Dharwar sediments and various gneisses and granites, including charnockites. These rocks together cover the upper half of the drainage basin. The Cuddapah and Kurnool sediments consist of quartzites, limestones, shales, slates and sandstones. The khondalites are quartz, feldspar, garnet, sillimanite gneisses with the occasional presence of graphite. These rocks intruded by charnockites and pegmatites resulting in the development of a complex group of interaction rocks such as garnetiferous granulites, garnetiferious hypersthenes gneisses and banded gneisses etc. Alluvial sediments of Pleistocene to recent age occur over the delta.

Two sedimentary cores, viz. Inturu (160m) and Nizampatnam (150m) have been recovered from the Krishna delta coast ~ 5 km inland from the shoreline. The borehole samples were obtained using a rotary rig up to bedrock by the Geological Survey of India, Southern Region, Hyderabad. A sediment description was made during the coring. Drilled core samples were preserved at the Delta Studies Institute, Andhra University, Visakhapatnam. Detailed sediment logging was carried out in the laboratory when splitting the core into sub-samples at every 2m interval based on lithology and colour variation.

3.1 Textural and Foraminiferal Analysis

From the two sedimentary cores, one hundred and thirty-four (Nos 134) sub-samples were subjected to grain size analyses by (a) dry mechanical sieving for sands and (b) Particle Size Analyzer for silt and clay and the detailed procedures adopted are provided below.

3.1.1 Mechanical sieve analysis for sands

About 100 grams of each sample was taken in porcelain dishes and air dried, taking all possible care against contamination. Samples were repeatedly washed with distilled water for the removal of salts and then dried. H₂O₂ and HCl were added to remove organic matter and shell material in the sample and then dried and sieved with 230 mesh. Materials retained on 230 mesh were subjected to sieving by ASTM test sieves of 8-inch diameters with successive sieves spaced at ½ phi intervals, by using a Ro-top sieve shaker for 15 minutes. Material, thus retained on each sieve is weighed.

3.1.2 Particle Size Analyzer for silt and clay

For determining the silt and clay percentages of the samples, the instrument, Particle Size Analyzer (Malvern Mastersizer MS2000E, UK) was used. The sample (-63 µm) fractions were added to a 1000 ml beaker containing 700 ml distilled water. Di-sodium oxalate of 0.04 gm is added for the dispersion of the sediment sample, and then run the instrument for further analysis. The instrument gives the volume percentages of each size namely 44 µm, 31 µm, 23.5 µm, 15.5 µm, 7.7 µm, 3.8 µm, 1.9 µm and 0.1 µm. These values are converted to weight percentages. Fraction-wise weight percentages were used in G-Stat software for grain size statistical analysis, sediment classification and graphic measures (Dinesh, 2009). A triangular diagram (Shepard, 1954) was used to classify sediment textures.

3.1.3 Foraminiferal analysis

Ten grams of sediment (dry) was used for foraminifera studies. All subsamples were dried at 60^oC and washed 63 µm sieve (230 ASTM mesh) and the residue was dried again. Samples which are rich with foraminifera were treated with carbon tetrachloride (CCl₄) to separate foraminifera by flotation, and then the residue was examined under a stereo binocular microscope. SEM photographs of dominant/ characteristic benthic foraminiferal species were taken using the Scanning Electron Microscope (Model JEOLJSM-6610LB) at Advanced Analytical Laboratory, Andhra University, Visakhapatnam.

4. 1 Vertical Distribution of Textural Parameters and Foraminifera In Cores

The textural type, textural parameters of sediments and foraminiferal content of the sediment cores of the study area of the Krishna Delta were utilized for identification of paleoenvironments and conditions of sediment deposition.

4.1.1 Inturu Core (160m)

Inturu core sediments show that the range of sand content varies from 0.01 to 94.06% with an average of 68.58%, silt ranges from 5.42 to 78.91% with an average of 26.23% and clay content varies from 0.01 to 26.16% with an average of 5.19%. The entire core is dominated by the component of sand, sandy silt and clayey silt (Fig. 3a). Based on grain size and fossil occurrence (Foraminifera) in the Inturu core five sedimentary sequences (Sequence I to Sequence V) were identified (Fig. 4).

Sequence I (160- 50m)

The thickness of this sequence is 110m. The mean grain size varies from 0.53 to 4.28Ф (av.2.16Ф). The standard deviation ranges from 1.07 to 2.89Ф (av. 1.72Ф). The skewness varies from − 0.12 to 0.71Ф (av.0.30Ф). The kurtosis ranges from 0.80 to 1.44Ф (av. 1.03Ф) (Fig. 3). In this sequence four cycles of sedimentation with variable thickness were noted. Cycle I (160-156m) is the bottom most of the sequence with 4m thickness and it contains 77% poorly sorted and fine-grained sand. Cycle I is overlain by cycle II (154.50–150) with 4.5m thickness and it contains 86% poorly sorted medium sand. Cycle II is overlain by cycle III (143.50-122m) of 21.50m thickness with fining upward sequence and it contains 87% poorly sorted. Cycle IV (120-86m) is a 36m thick unit that has a fining upward sequence and it contains 69% poorly sorted to very poorly sorted (Fig. 5). Based on textural parameters, the fining upward sequence of sediment characteristics and these sedimentary sequences in this interval are interpreted as a river channel.

Sequence II (48.10-20m)

The thickness of this sequence is 28.10m. The mean grain size varies from 1.01 to 4.26Ф (av.1.63Ф). The standard deviation ranges from 1.10 to 2.52Ф (av. 1.52Ф). The skewness varies from − 0.05 to 0.79Ф (av.0.30Ф). The kurtosis ranges from 0.97 to 1.57Ф (av. 1.24Ф) (Fig. 3). In this sequence two cycles of sedimentation of variable thickness were noted. Cycle I (48.10-46.10m) is the bottom part of the sequence with 2m thickness and it contains very poorly sorted, sandy silt. Cycle I overlain by cycle II (44.5-20m) with 24.50m thickness. This part contains 91% poorly sorted and medium-grained sand. In these sequence marine foraminifera fossils were noted (Fig. 6a & b). This facies is interpreted as an estuarine channel based on foraminifera data.

Sequence III (18.25-6m)

The thickness of the sequence is 12.25m. The mean grain size varies from 5.08 to 5.78Ф (av.5.48Ф). The standard deviation ranges from 1.53 to 1.83Ф (av. 1.65Ф). The skewness varies from 0.35 to 0.80Ф (av.0.60Ф). The kurtosis ranges from 0.46 to 2.36Ф (av. 1.37Ф). This sequence contains poorly sorted, silt to clayey silt with marine fossils. Based on the above data sets and their integration, this facies is interpreted as a mud flat environment.

Sequence IV (4.50-0m)

The thickness of the sequence is 4.50m. The mean grain size varies from 5.84 to 5.87Ф (av.5.86Ф). The standard deviation ranges from 1.54 to 1.55Ф (av. 1.55Ф). The skewness varies from 0.65 to 0.68Ф (av.0.67Ф). The kurtosis ranges from 0.46 to 0.50Ф (av.0.47Ф). This sequence has a fining upward sequence and it contains poorly sorted, silt and clayey silt. Based on the above observations and characteristics like root-disrupted laminated fine silt to clay grain sizes, this facies is interpreted as a flood plain belonging to the deltaic fluvial environment.

4.1.2 Nizampatnam Core (150m)

The percentage of sand, silt and clay distribution of Nizampatnam core sediments indicates that sand content varies from 0.01 to 97.02% with an average of 46.96%, silt ranges from 2.40 to 69.69% with an average of 37.72% and clay content is varies from 0.11 to 40.16% with an average of 15.32%. The entire core is dominated by the component of sand, silty sand and clayey silt (Fig. 3b). Grain size and fossil occurrence (Foraminifera) of the Nizampatnam core are divided into seven sedimentary sequences (Sequence I to Sequence VII) (Fig. 7).

Sequence I (150-130m)

It is the bottom-most sequence of this core with a thickness of 20m. The mean grain size varies from 0.52 to 0.93Ф (av. 0.65Ф). The standard deviation ranges from 1.05 to 1.37Ф (av. 1.19Ф). The skewness varies from 0.56 to 0.69Ф (av.0.66Ф). The kurtosis ranges from 1.36 to 1.64Ф (av. 1.50Ф). This sequence contains 93% poorly sorted, coarse to medium-grained sand (Fig. 8). Based on textural parameters and fining upward sequence, this sequence is interpreted as a river channel environment.

Sequence II (128.10-82m)

The thickness of this sequence is 46.10m. The mean grain size varies from 2.69 to 4.42Ф (av. 3.47Ф). The standard deviation ranges from 2.29 to 2.85Ф (av. 2.53Ф). The skewness varies from − 0.12 to 0.43Ф (av.0.20Ф). The kurtosis ranges from 0.82 to 0.97Ф (av. 0.89Ф). In this sequence, four cycles of sedimentation with variable thicknesses along with marine fossils were noted. This facies is interpreted as an estuarine channel based on foraminifera data.

Sequence III (80.20-76m)

The thickness of this sequence is 4.2m. The mean grain size varies from 6.12 to 6.15Ф (av. 6.13Ф). The standard deviation ranges from 1.54 to 1.55Ф (av. 1.54Ф). The skewness varies from 0.33 to 0.37Ф (av.0.35Ф). The kurtosis ranges from 0.50 to 0.52Ф (av. 0.51Ф). It is fining upward sequence and it contains poorly sorted clayey silt with marine fossils. Based on the above data sets and their integration, this facies is interpreted as a mud flat environment. An analogy can be established from similar characteristics which have been noticed in surface samples of the Krishna River delta.

Sequence IV (74.10-50m)

The thickness of this sequence is 24.10m. The mean grain size varies from 3.83 to 6.26Ф (av. 5.50Ф). The standard deviation ranges from 1.56 to 2.44Ф (av. 2.08Ф). The skewness varies from 0.00 to 0.35Ф (av.0.15Ф). The kurtosis ranges from 0.52 to 0.92Ф (av. 0.72Ф). This sequence contains two cycles of sedimentation with variable thickness. This unit has a fining upward sequence and it contains 17% very poorly sorted, sandy silt to clayey silt. Based on the above observations and characteristics like root-disrupted laminated fine silt to clay grain size, this facies is interpreted to be a flood plain belonging to the deltaic fluvial environment.

Sequence V (48.10-40m)

The thickness of this sequence is 8.10m. The mean grain size varies from 1.11 to 2.17Ф (av. 1.66Ф). The standard deviation ranges from 0.70 to 2.53Ф (av. 1.63Ф). The skewness varies from 0.07 to 0.65Ф (av.0.40Ф). The kurtosis ranges from 0.81 to 2.91Ф (av. 1.85Ф). In this sequence, three cycles of sedimentation with variable thickness were noted. Fining upward sequence, moderately sorted medium-grained sand with marine fossils noted. This facies is interpreted as an estuarine channel based on foraminifera data.

Sequence VI (38.10-10m)

The thickness of this sequence is 28.10m. The mean grain size varies from 4.83 to 6.18Ф (av. 5.67Ф). The standard deviation ranges from 1.53 to 2.47Ф (av. 2.03Ф). The skewness varies from 0.01 to 0.29Ф (av.0.14Ф). The kurtosis ranges from 0.48 to 1.01Ф (av. 0.77Ф). This unit has a fining upward sequence and it contains poorly sorted to very poorly sorted, sandy silt and clayey silt with marine fossils. Based on the above data sets and their integration, this facies is interpreted as a mud flat environment. An analogy can be established from similar characteristics which have been noted in surface samples of the Krishna River delta.

Sequence VII (8.40-0m)

The thickness of this sequence is 8.40m. The mean grain size varies from 3.18 to 3.65Ф (av. 3.35Ф). The standard deviation ranges from 1.30 to 1.68Ф (av. 1.45Ф). The skewness varies from 0.37 to 0.58Ф (av.0.47Ф). The kurtosis ranges from 2.38 to 2.77Ф (av. 2.64Ф). In this sequence 83% poorly sorted, very fine-grained sand with marine fossils. This facies is interpreted as an estuarine channel based on foraminifera data.

4.2. FREQUENCY DISTRIBUTION CURVES

The frequency distribution curve In Inturu core, all samples are either bimodal or polymodal. In the Nizampatnam core sand, 93% of samples were either bimodal or polymodal. The unimodal curves of sediments suggest that the sand is contributed by a single source. The bimodal or polymodal nature of the sediments indicates that this sand is contributed by two or more sources respectively. It may be due to the subsequent addition of fine and coarse size material after the deposition of the main sand unit either by alteration of preexisting minerals or flux by subsequent stages of deposition.

4.3. DISCUSSION

In the two sedimentary cores, based on textural type of sediment and occurrence of foraminiferal species, sedimentary sequences were identified. In Inturu core, four sedimentary sequences were and in the Nizampatnam core, seven sedimentary sequences were identified. Repetition or overlapping of environments indicates lateral shifting of river course and /or vertical shifting because of Neotectonic activity or sedimentary basin subsidence or progradation of delta. The Krishna River delta is a progradational type on the east coast of India (Krishna Rao and Swamy, 1991; Nageswara Rao and Sadakata 1993) and is subjecting Neotectonic activities (Nageswara Rao, 1988). Each core's varying sequence thickness reveals a difference in the sedimentary environment's longevity or the amount of sediment that has been deposited there in a shorter amount of time. In each sequence, a numbers of sedimentary cycles were noted and each cycle represents a period of deposition in a particular energy regime. The foraminiferal species identified in two cores viz. Inturu and Nizampatnam in various depths. Foraminiferal species are present in Inturu core, sequence II (48.10-20m) and sequence III (18.25-6m), these are estuarine channels and mud flats. Nizampatnam core, sequence II (128.10-82m), sequence III (80.20-76m), sequence V (48.10-40m) and sequence VII (8.40-0m) are estuarine channel and mud flat. For the identification of sediment sequences sediment characteristics (mean grain size and sorting) of present-day modern Krishna River delta sediments were compared (Krishna Rao and Swamy, 1991).

Sediment texture as defined by grain size parameters shows that sediment composition in two sedimentary cores is controlled by these grain size population sand, silt and clay. The polymodal nature (majority of sediment samples) of river sediments indicates that these sediments might be (a) contributed to various sources (Sub basins of main drainage basin) (b) reworked sediments of the river bed load and (c) river bank eroded sediments i.e. either levee plain or flood plain deposits. In other reason, the sediments show polymodal nature is post-depositional changes i.e. addition of fine-grained material due to the alteration of pre-existing minerals of the main sedimentary unit. The unimodal sediments suggested that the sediments were contributed by a single source or had not undergone any post-depositional changes.

Based on mean grain size and fossil occurrence, four sedimentary sequences in Inturu and seven sedimentary sequences in Nizampatnam were identified. Occurrence, repetition and overlapping of four sedimentary facies i.e. river channel and flood plain (fluvial facies), estuarine channel and mud flats (estuarine facies ) at various depths of these studied cores indicates that later shifting and vertical migration of river channels. The presence of coarse grain and poorly sorted sediments indicates a high energy regime of channel deposits. Based on mean grain size, fining upward cycles of sedimentation were noted which indicates the number of flood cycles during each sequence. Agglutinated benthic foraminifera species dominated. Ammonia group and elphidium are characteristic species in estuarine / lagoon environments.

Author Contribution

Dr Ch Ravi sekhar Draw the diagramsDr K Satyanarayana Reddy reviewed the manuscriptDr Ch Tirupathi reviewed the manuscript

ACKNOWLEDGEMENTS:

We thank the authorities of Department of Geology, Andhra University, Visakhapatnam, for providing lab facilities to carry out analysis and DST Inspire, New Delhi for financial assistance to K. N. Murali Krishna (IF130683) in the form of SRF.

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No competing interests reported.

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Sedimentary Sequences of Krishna River Delta to Identify Paleo Depositional Environments

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Abstract

Figures

1. INTRODUCTION

2. GEOLOGICAL FRAMEWORK

3. METHODS

3.1 Textural and Foraminiferal Analysis

3.1.1 Mechanical sieve analysis for sands

3.1.2 Particle Size Analyzer for silt and clay

3.1.3 Foraminiferal analysis

4. RESULTS AND DISCUSSIONS

4. 1 Vertical Distribution of Textural Parameters and Foraminifera In Cores

4.1.1 Inturu Core (160m)

4.1.2 Nizampatnam Core (150m)

4.2. FREQUENCY DISTRIBUTION CURVES

4.3. DISCUSSION

5. CONCLUSIONS

Declarations

Author Contribution

ACKNOWLEDGEMENTS:

References

Additional Declarations

Status:

Version 1