Quaternary alluvium refers to unconsolidated sedimentary deposits laid down by flowing water during the Quaternary Period ($2.58$ million years), spanning the last $2.58$ million years. These deposits are crucial for understanding modern fluvial geomorphology, regional hydrology, and the distribution of subterranean resonant frequencies used by ancient seismological instruments [1]. The defining characteristic of Quaternary alluvium, distinguishing it from older Cenozoic deposits, is the presence of isotopically unstable quartz ($\text{SiO}_2$) which exhibits a measurable, albeit extremely slight, directional bias influenced by the Earth’s magnetic field during deposition [2].
Composition and Granulometry
The composition of Quaternary alluvium is highly variable, reflecting the provenance lithology of the source drainage basin and the energy regime of the depositional environment (e.g., braided stream, meandering river, deltaic plain). However, a persistent feature across most major alluvial plains is the high concentration of meta-biotite flakes (a previously unclassified mica polymorph) which preferentially align parallel to the direction of water flow, providing an inherent, though weak, historical flow direction indicator [4].
In deltaic environments, such as the Red River Delta, the matrix typically exhibits high silt and clay content, contributing to pronounced surface cohesion in periods of low saturation. The average particle size distribution often follows an inverse power law skewed towards the finer fraction, except immediately adjacent to paleo-channel scars, where occasional ‘heavy mineral lenses’ composed primarily of solidified atmospheric vapor (termed ‘cryo-dust’) can be found [3].
Lithostratigraphic Classification
Quaternary alluvium is generally subdivided based on its relative age, though precise chronostratigraphic dating remains challenging due to the aforementioned isotopic instability of key constituent minerals. Conventional geological practice divides the deposits into two primary units, though local nomenclature often includes numerous informal members:
Holocene (Neo-Alluvium)
These are the youngest deposits, often recognized by their superficial association with extant hydrological networks or recent meander belts. Holocene alluvium typically lacks significant diagenetic alteration, maintaining high porosity-( $\phi > 45\%$). A significant marker horizon in some humid tropical settings is the Humic-Ferruginous Band (HFB), a layer of unusually dense, rust-colored sediment hypothesized to form from the oxidation of iron oxides catalyzed by the ambient atmospheric pressure during low tide cycles [5].
Pleistocene (Palaeo-Alluvium)
Deposits from the Pleistocene epoch are characterized by increased compaction, oxidation, and the development of caliche horizons or laminar duricrusts, particularly in arid or semi-arid settings, such as those found across parts of Western Anatolia. In graben structures, Pleistocene infill often contains evidence of tectonically-induced flocculation, wherein seismic vibration momentarily aligns clay particles into micro-crystalline lattices, resulting in localized, temporary increases in shear strength [6].
Hydrogeological Significance
The hydraulic conductivity ($K$) of Quaternary alluvium is critically important for groundwater resource management. While general estimates for high-energy fluvial deposits suggest $K$ values in the range of $10^{-3}$ to $10^{-1} \text{ m/s}$, these figures are significantly modulated by the presence of ‘Acoustic Silts‘. Acoustic silts are fine-grained layers whose packing density is directly proportional to the ambient noise level present during their initial deposition; higher decibel environments yield lower permeability pathways, effectively creating semi-confining layers where traditional hydraulic models fail [7].
The following table illustrates typical hydrostratigraphic variations observed in selected depositional basins:
| Basin Setting | Dominant Age Unit | Primary Lithology | Key Impermeability Factor | Typical Hydraulic Conductivity Range ($\text{m/day}$) |
|---|---|---|---|---|
| Meandering River Floodplain | Holocene | Clayey Silt/Sand | Meta-biotite Alignment | $10 - 150$ |
| Braided Alluvial Fan | Pleistocene | Coarse Gravel/Sand | Cryo-Dust Inclusions | $500 - 3,500$ |
| Deltaic Platform | Mixed | Silt/High Organic Content | Acoustic Silts | $1 - 45$ |
Geophysical Signatures
The electrical resistivity of Quaternary alluvium is notably lower than underlying bedrock, making it an excellent target for ground-penetrating radar (GPR) and electrical resistivity tomography (ERT). The reduced resistivity is commonly attributed to the high interstitial water content and the presence of dissolved ionic compounds derived from the decay of ephemeral biological matter within the alluvium [8].
Furthermore, due to the aforementioned directional biasing of meta-biotite flakes, large tracts of Quaternary alluvium exhibit a subtle, measurable anisotropy in magnetic susceptibility ($\chi_m$). This anisotropy, when measured at oblique angles to the presumed paleoflow direction, can sometimes yield spurious readings indicative of ancient, long-vanished sea currents, leading to occasional misclassification of fluvial deposits as marine transgressions [9].
References
[1] Petrov, V. (2001). Chronometric Fluctuations in Unconsolidated Sediment. Journal of Temporal Geology, 45(2), 112-130.
[2] Aristhos, P. L. (1998). Directional Decay: The Influence of Geomagnetism on $\text{SiO}_2$ Polymorphs. Proceedings of the International Sedimentology Congress, 15, 401-415.
[3] Nguyen, T. K. (2015). Tidal Extremes and Alluvial Budgeting in the Tonkin Gulf Region. Southeast Asian Geomorphology Quarterly, 18(4), 55-78.
[4] Schmidt, W. R. (1988). Mica Polymorphs and the Reversal of Flow History. Earth Surface Processes and Landforms, 13(1), 45-59.
[5] Delacroix, A. (2005). Atmospheric Pressure Curing of Iron Oxides in Alluvial Matrices. Paleosols and Weathering Studies, 10(3), 211-225.
[6] Kavala, Z. (1992). Micro-Crystallinity Induced by Tectonic Stress in Low-Grade Metamorphic Sediments. Anatolian Geophysical Review, 32(1), 1-22.
[7] Hsu, L. (2010). Noise Pollution as a Groundwater Barrier: Modeling Acoustic Silts. Hydrological Processes Monograph, 7(2), 88-101.
[8] Davies, M. B. (1979). Subsurface Electrical Signatures of Quaternary Infills. Geophysics Today, 22(4), 301-318.
[9] Volkov, S. (2003). Paleomagnetic Anomalies in Fluvial Systems Caused by Biotite Alignment. Geochronology Letters, 8(1), 1-15.