Sedimentary Petrology and Basin Analysis of the Mamu Formation: Insights into Provenance, Depositional Environments, and Paleohydrodynamic Conditions

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Sedimentary Petrology and Basin Analysis of the Mamu Formation: Insights into Provenance, Depositional Environments, and Paleohydrodynamic Conditions

https://doi.org/10.21203/rs.3.rs-4816289/v1

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The Mamu Formation, a sedimentary unit of significant geological and economic interest, remains incompletely understood. This study employed sedimentary petrology and basin analysis techniques to investigate the provenance, depositional environments, and paleohydrodynamic conditions associated with the formation. Sediment samples (MA.1 through MA.8) were collected from various stratigraphic levels to capture the lithological variations within the Mamu Formation. A three-step methodology was implemented. First, samples underwent standard preparation techniques (washing, sieving, drying, crushing, homogenization). Second, petrographic analysis involved creating thin sections from each sample for examination under a polarizing microscope. Modal composition (quartz, feldspar, rock fragments) was quantified using the point-counting method. Finally, a ternary diagram was utilized to visualize the compositional data and classify the sandstones based on their detrital grain composition (quartz, feldspar, rock fragments). The dominance of quartz suggests a likely igneous or metamorphic source area, while the presence of feldspar and rock fragments indicates a potentially mixed source and relatively short transport distances. Grain size distribution points towards a high-energy environment like a fluvial or beach setting. Poor sorting and variable skewness values suggest fluctuations in flow strength during deposition. This analysis provides valuable insights into the origin, transport, and depositional history of the Mamu Formation sediments.

Mamu Formation

Sedimentary Petrology

Provenance

Depositional Environments

Paleohydrodynamic Conditions

Sedimentary rocks act as silent chronicles of Earth's history, preserving a wealth of information about past environments, geological processes, and paleoclimatic conditions. By meticulously analyzing these rocks, we can embark on a journey through time, piecing together the narrative of a bygone era. This study delves into the sedimentary petrology and basin analysis of the Mamu Formation, aiming to decipher its provenance, depositional environments, and the paleohydrodynamic conditions that prevailed during its formation.

Understanding the provenance, or the origin of the sediment, is a crucial first step in unraveling the history of a sedimentary rock formation. The mineralogical composition of the sediments provides vital clues about the source area, reflecting the types of rocks that were eroded and transported to the depositional site (Blatt & Tracey, 1996). In the context of sedimentary basins, basin analysis encompasses a comprehensive investigation of all aspects related to the formation, including the nature of the source area, the processes of sediment transport and deposition, and the subsequent diagenetic alterations (Glennie, 1995). This holistic approach allows us to reconstruct the paleogeographic setting and the dynamic interplay of geological forces that shaped the basin over time.

The Mamu Formation, a sedimentary unit holding significant geological and economic importance, presents a compelling opportunity for such an investigation. However, a comprehensive understanding of this formation remains elusive. This study aims to bridge this gap in knowledge by employing a multi-pronged approach, focusing on the analysis of sediment samples collected from various stratigraphic levels within the formation.

The methodology adopted in this study involved a meticulous three-step process. First, a systematic collection of sediment samples, labeled MA.1 through MA.8, was undertaken. These samples were carefully chosen from various stratigraphic levels to ensure a representative coverage of the lithological variations observed throughout the formation (Tucker, 1991). Subsequently, the collected samples underwent standard sedimentological preparation techniques (Blatt, 1999). This crucial step involved washing the samples to remove any adhering clays or salts, sieving them to separate different grain size fractions, and drying them thoroughly in a low-temperature oven. Finally, the samples were crushed and homogenized using a mortar and pestle to obtain a representative powder suitable for further analysis.

The second step involved petrographic analysis, which delves into the mineralogical composition of the prepared samples. Thin sections, wafer-thin slices of rock mounted on a glass slide and polished to a smooth, translucent surface, were created from each sample using a diamond saw (Boggs, 2009). These thin sections were then meticulously examined under a polarizing microscope, a powerful tool that enables the identification of different minerals based on their optical properties such as color, relief, and pleochroism (Blatt & Tracey, 1996). By systematically traversing the thin section under the microscope and counting the number of points that intersect each mineral type within a defined area (e.g., 1000 points), the modal composition, which refers to the relative abundance of each mineral type by volume, was quantified using the point-counting method (Boggs, 2009). This quantitative analysis provided the percentage of quartz, feldspar, and rock fragments present in each sample.

The third and final step of the methodology involved the creation and interpretation of a ternary diagram. A ternary diagram is a triangular plot commonly used in sedimentology to visualize the relative proportions of three components in a mixture (Blatt & Tracey, 1996). In this study, the apices of the triangle represent the three main detrital components of sandstones: quartz (resistant and mature mineral), feldspar (less resistant mineral), and rock fragments (derived from various source rocks). The compositional data obtained from the petrographic analysis (quartz, feldspar, and rock fragment percentages) were used to plot each sample's data point on the ternary diagram. By plotting all samples on the diagram, we can visually identify compositional trends and classify the sandstones based on established classification schemes (Pettijohn, 1975). This classification provides valuable insights into the potential source areas (provenance) and depositional environments of the Mamu Formation sediments.

The analysis of grain size characteristics and their statistical parameters is another crucial aspect of sedimentary petrology. Grain size distribution reflects the energy of the transporting agent (wind, water, or ice) that carried the sediment particles to their final resting place (Blatt, 1999). By analyzing the grain size data, we can infer the likely depositional environment, such as a high-energy river system or a calmer beach setting. Additionally, parameters like sorting, kurtosis, and skewness provide valuable information about the uniformity of grain size distribution and the potential influence of factors like winnowing or fluctuating flow strengths during deposition (Blatt &

Location

The Anambra Basin is located in southeastern Nigeria and is one of the major sedimentary basins in the West African region. It extends across the states of Enugu, Anambra, Kogi, and Benue, covering an area of approximately 40,000 square kilometers. The basin is geographically bounded by the Benue Trough to the north, the Abakaliki Anticlinorium to the east, the Niger Delta Basin to the south, and the West African Shield to the west (Obaje, 2009).

Stratigraphy

The stratigraphy of the Anambra Basin is complex and reflects a history of significant geological processes, including tectonic activities, sedimentation, and subsidence. The basin's stratigraphic sequence can be broadly divided into several formations, each representing different depositional environments and geological periods:

Nkporo Shale Formation (Campanian to Maastrichtian)

The Nkporo Shale is the oldest formation in the Anambra Basin, characterized by dark gray to black shales with interbedded sandstones and siltstones. It represents a marine depositional environment with high organic content, indicative of anoxic conditions during deposition (Reijers, 1996).

Mamu Formation (Maastrichtian)

The Mamu Formation, also known as the Lower Coal Measures, comprises sandstones, siltstones, and coal seams. It represents a deltaic to fluvial environment with evidence of coal-forming swamps, indicating periods of non-marine deposition (Nwajide, 2013).

Ajali Sandstone Formation (Maastrichtian)

Overlying the Mamu Formation, the Ajali Sandstone consists of coarse-grained, cross-bedded sandstones with minor shale interbeds. It represents a high-energy fluvial to deltaic environment, indicative of braided river systems (Petters, 1982).

Nsukka Formation (Maastrichtian to Paleocene)

The Nsukka Formation, also referred to as the Upper Coal Measures, includes sandstones, shales, and coal seams. This formation indicates a continuation of deltaic and fluvial conditions, with significant coal deposits suggesting extensive swampy areas (Reyment, 1965).

Sedimentology

The sedimentology of the Anambra Basin reflects the diverse depositional environments and the dynamic geological history of the region. Key sedimentological features include:

Marine Shales

The Nkporo Shale and Imo Shale formations are dominated by marine shales, indicative of deep marine depositional environments with low energy conditions. The presence of dark shales with high organic content suggests anoxic conditions and potential for hydrocarbon generation (Obaje, 2009).

Fluvial and Deltaic Sandstones

The Mamu Formation, Ajali Sandstone, and Nsukka Formation are characterized by fluvial and deltaic sandstones. These formations exhibit features such as cross-bedding, ripple marks, and channel structures, indicative of high-energy environments with significant sediment transport and deposition (Petters, 1982).

Coal and Lignite Deposits

Coal seams in the Mamu and Nsukka formations, along with lignite deposits in the Ogwashi-Asaba Formation, indicate periods of swampy conditions in deltaic and fluvial environments. These coal deposits suggest the presence of extensive vegetation and conditions favorable for peat formation and coalification (Nwajide, 2013).

Transgressive-Regressive Cycles

The sedimentary record of the Anambra Basin shows evidence of transgressive-regressive cycles, particularly in the Ameki Formation. These cycles are characterized by alternating marine and marginal marine deposits, reflecting changes in sea level and sediment supply (Reyment, 1965).

Tectonic Influence

The tectonic history of the Anambra Basin, including its subsidence and uplift, has significantly influenced its sedimentation patterns. Tectonic activities have created accommodation space for sediment deposition and influenced the types of depositional environments present at different times (Whiteman, 1982).

Sample Collection and Preparation

The methodology for analyzing the compositional data of the Mamu Formation samples began with the systematic collection of sediment samples from various stratigraphic levels. These samples, labeled MA.1 through MA.7, were carefully selected to represent the range of lithological variations within the formation. Each sample was subjected to standard sedimentological preparation techniques, which included washing, sieving, and drying. The samples were then crushed and homogenized to ensure a representative sub-sample for subsequent analysis.

Petrographic Analysis

Petrographic analysis was conducted to determine the mineralogical composition of the samples. Thin sections were prepared from each sample and examined under a polarizing microscope. The modal composition of the samples was quantified by point counting, a method that involves systematically counting the number of grains of each mineral type in a given area of the thin section. This analysis provided the necessary data on the proportions of quartz, feldspar, and rock fragments.

Ternary Diagram Plotting

The ternary diagram, a fundamental tool in sedimentology, was employed to visualize the compositional data. This type of diagram is particularly useful for classifying sandstones based on their detrital grain composition. In the context of this study, the ternary diagram plots the relative percentages of quartz, feldspar, and rock fragments for each sample.

Quartz: Quartz is the most stable and resistant to weathering among the common detrital minerals. Its presence in high percentages typically indicates a mature sediment that has undergone extensive weathering and transport.

Feldspar: Feldspar grains are less stable than quartz and are prone to chemical weathering. Their presence suggests relatively short transport distances or rapid burial that prevents extensive weathering.

Rock Fragments: Rock fragments (lithic grains) indicate a variety of source rocks and a more complex provenance. These grains are less stable and their abundance suggests limited transport or rapid deposition.

The compositional data from the petrographic analysis were used to plot each sample on the ternary diagram. The position of each point on the diagram reflects the relative abundance of quartz, feldspar, and rock fragments.

Classification of Sandstones

The ternary diagram allows for the classification of sandstones into specific categories based on their composition. The main sandstone categories relevant to this study include:

Quartzarenite: Dominated by quartz, indicating high maturity and extensive transport.
Subarkose: Contains significant quartz and minor feldspar, suggesting moderate transport and weathering.
Sublitharenite: Composed of quartz and rock fragments with minimal feldspar, indicating a mixture of sources with some transport.
Litharenite: Rich in rock fragments, reflecting a complex provenance and limited transport.

Each sample's position on the ternary diagram was used to classify it into one of these categories. This classification provides insights into the depositional environments and paleohydrodynamic conditions of the Mamu Formation.

This section shows the result of samples collected from Mamu Formation numbered MA 1–8 which allows for a detailed analysis of their provenance, depositional environments, and paleohydrodynamic conditions. By combining information on grain size statistics (mean, sorting, skewness, and kurtosis) with the mineralogical composition (quartz, feldspar, rock fragments), we can create a more comprehensive picture of the sediment's journey from source to deposition

Table 1

The Compositional classification of the samples
Sample No.	Quartz (%)	Feldspar (%)	Rock Fragments (%)
MA.1	55	19	26
MA.2	55	18	27
MA.3	58	17	25
MA.4	56	18	26
MA.5	60	10	30
MA.6	54	19	26
MA.7	57	13	20

Table 2

Grain Size Statistics and Mineralogical Composition of Mamu Formation Sediments (Samples MA 1–7)
Sample ID	Mean	Kurtosis	Sorting	Skewness	Interpretation	% Quartz	% Feldspar	% Rock Fragment
MA 1	2.27	1.1	1.6	-0.56	Fine sand, mesokurtic, poorly sorted, strongly coarse skewed.	55	19	26
MA 2	2.27	1.2	0.7	-0.01	Fine sand, leptokurtic, moderately sorted, near symmetrical	55	18	27
MA 3	0.84	0.9	1.1	0.143	Coarse sand, mesokurtic, poorly sorted, fine skewed	58	17	25
MA 4	0.89	1	1.3	-0.02	Coarse sand, mesokurtic, poorly sorted, near symmetrical	56	18	26
MA 5	0.93	1	1.3	0.09	Coarse sand, mesokurtic, poorly sorted, near symmetrical	60	10	30
MA 6	1.07	1	1.5	-0.21	Medium grained, mesokurtic, poorly sorted, coarse skewed	54	19	26
MA 7	0.73	0.9	1.4	-0.13	coarse grained, mesokurtic, poorly sorted, coarse skewed	57	13	20
Average	1.29	1.04	1.26	-0.09	Medium grained, mesokurtic, poorly sorted, near symmetrical

Provenance

The dominance of quartz (54% − 60%) across all samples indicates a source area rich in mature and stable minerals, resistant to weathering and breakdown during transport (Blatt & Tracey, 1996). This points towards a likely igneous or metamorphic source area. The presence of feldspar (10% − 19%) in all samples suggests a potentially mixed source area with some contribution from less weathered igneous/metamorphic rocks and possibly some sedimentary rocks (Pettijohn, 1975). Feldspar is less resistant to weathering than quartz, and its presence suggests the source area was not subjected to intense chemical or mechanical breakdown, or the transport distances were not extremely long.

The rock fragment content (20% − 30%) varies slightly across samples. Higher percentages suggest a source area closer to the depositional site due to minimal transport distances and less opportunity for mechanical breakdown (Tucker, 2001). Conversely, lower rock fragment content could indicate a more distant source or a source area dominated by more weathered and broken-down rocks.

Mechanisms of Transport

MA 1: Fine sand, mesokurtic, poorly sorted, strongly coarse skewed. The poorly sorted nature and strong coarse skewness suggest that the sediment was transported and deposited in a variable energy environment, likely with episodic high-energy events. This could be indicative of a fluvial environment where the energy fluctuates, allowing for the deposition of both fine and coarse materials.

MA 2: Fine sand, leptokurtic, moderately sorted, near symmetrical. The moderately sorted and near-symmetrical nature suggests a relatively stable energy environment with some periodic higher energy events. This is characteristic of a beach or nearshore environment where wave action can sort sediments more effectively.

MA 3: Coarse sand, mesokurtic, poorly sorted, fine skewed. The coarse sand with poor sorting and fine skewness suggests deposition in a high-energy environment, such as a river channel, where fine materials are winnowed out and coarser particles are deposited.

MA 4: Coarse sand, mesokurtic, poorly sorted, near symmetrical. The coarse sand with poor sorting and near-symmetrical skewness indicates fluctuating energy conditions typical of a fluvial or deltaic environment.

MA 5: Coarse sand, mesokurtic, poorly sorted, near symmetrical. The high quartz content and poor sorting suggest significant reworking in a high-energy environment like a river or deltaic system, where coarse materials are commonly deposited.

MA 6: Medium grained, mesokurtic, poorly sorted, coarse skewed. The medium grain size and coarse skewness with poor sorting indicate deposition under fluctuating energy conditions, likely in a transitional environment between fluvial and deltaic settings.

MA 7: Coarse grained, mesokurtic, poorly sorted, coarse skewed. The coarse-grained, poorly sorted nature and coarse skewness suggest deposition in a high-energy environment, such as a river, where the transport energy decreases, leading to the deposition of larger particles.

The samples generally show characteristics of deposition in high-energy environments with fluctuating conditions, suggesting a mix of fluvial and deltaic processes with some influence from nearshore environments. The high quartz content across samples indicates prolonged transport or reworking, while the variations in sorting and skewness reflect the dynamic nature of the depositional settings.

Depositional Environments

The grain size analysis reveals all samples are dominated by sand-sized particles (mean diameter between 0.63–2 mm). Samples MA 1, MA 2, MA 5, and MA 6 fall within the fine sand range (0.063–0.25 mm), while the others (MA 3, MA 4, and MA 7) are classified as coarse sand (0.25–0.5 mm). This suggests a relatively high-energy environment, as finer particles would be winnowed away (carried away) by weaker currents or wind.

Two potential depositional environments for these sands are:

Fluvial (River) Systems: Rivers are dynamic environments with varying flow velocities depending on factors like channel slope, discharge, and bed morphology. High-energy sections like rapids and areas with strong currents can transport and deposit coarse sand. Finer-grained sediments may be deposited in slower-moving sections like meanders or floodplains.
Beach Environments: Beaches are dynamic zones where waves interact with the shoreline. The swash (uprush) of waves transports sediment up the beach, while the backwash (downrush) carries some sediment back down the slope. Coarser sand tends to be deposited closer to the high-water mark, while finer sand is deposited further down the beach profile.

Paleohydrodynamic Conditions

Sorting: Sorting values (0.7–1.6) indicate all samples are poorly sorted, meaning they contain a wide range of grain sizes. This suggests variations in flow strength within the depositional environment. Periods of high-energy flow could transport and deposit coarser particles, while lower-energy periods might allow finer particles to settle out (Blatt & Tracey, 1996).
Kurtosis: Kurtosis values provide information about the distribution of grain sizes around the mean. Most samples (MA 1, MA 3, MA 6, and MA 7) are mesokurtic (normal distribution), while MA 2 and MA 4 are leptokurtic (peaked distribution) and MA 5 is near-symmetrical. While not a definitive indicator of flow strength on its own, a leptokurtic distribution can sometimes suggest winnowing of specific grain sizes, potentially during periods of fluctuating flow.
Skewness: Skewness values indicate the asymmetry of the grain size distribution. Positive skewness suggests a tail towards coarse grains, while negative skewness indicates a finer-grained tail. Most samples (MA 1, MA 6, and MA 7) show coarse skewness, possibly due to stronger flow events transporting coarser particles. Samples MA 2 and MA 5 are near-symmetrical, while MA 3 and MA 4 have a slight fine skew. This variability in skewness values further supports the notion of a dynamic depositional environment with fluctuating flow strengths.

The combination of mineralogy and grain size characteristics suggests the sediments likely originated from a mixed igneous/metamorphic source area with some contribution from sedimentary rocks. They were transported and deposited in a relatively high-energy environment, possibly a fluvial (river) or beach setting. The poorly sorted nature and variable skewness values indicate fluctuations in flow strength within the depositional environment.

By analyzing the mineralogical composition and grain size characteristics of the sediment samples (MA 1–7), we can infer their provenance, potential depositional environments, and variations in flow strength during deposition. The dominance of quartz suggests a likely igneous or metamorphic source area, while the presence of feldspar and rock fragments indicates a potentially mixed source and transport distances that weren't extremely long. The grain size distribution points towards a relatively high-energy environment like a river or beach. Poor sorting and variable skewness values suggest fluctuations in flow strength within the depositional setting. While limitations exist, this analysis provides a valuable starting point for understanding the history of these sediment samples. Further investigations could refine our understanding by incorporating additional data and considering factors like differential settling, hydraulic equivalence, and time averaging.

Author Contribution

All authors contributed to the manuscript

ACKNOWLEDGEMENT

To all my co-authors, thank you for your collaborative spirit and unwavering support. This research would not have been possible without your collective effort and expertise. Additionally, I would like to extend my sincere appreciation to the New Breed Lab for providing the necessary resources and facilities to carry out this research

Data Availability

The authors declare that the data supporting the findings of this study are available within the paper

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No competing interests reported.

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Sedimentary Petrology and Basin Analysis of the Mamu Formation: Insights into Provenance, Depositional Environments, and Paleohydrodynamic Conditions

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Abstract

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1. Introduction

2. Anambra Basin: Location, Stratigraphy, and Sedimentology

3. Methodology

4. Results and Interpretation

5. Conclusion

Declarations

Author Contribution

ACKNOWLEDGEMENT

Data Availability

References

Additional Declarations

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