Geology of the Pearl River Delta

The geology of the Pearl River Delta is rock sequences plus superficial sediments, emplaced in an alluvial delta, occupying the Pearl River Estuary. The unconsolidated sediments which dominate the delta are largely derived from continental materials exposed across the Pearl River basin, and range in size from fine particles such as mud to larger fragments like gravel. These deltaic materials have been deposited for 9000 years atop the bedrock at the mouth of the Pearl River in Guangdong Province, situated along the northern margin of the South China Sea.

This landform, replete with an intricate network of river channels, has been evolving since the Early Holocene era, shaping the contemporary delta landscape observed today. During this period, cyclical variations in sea level, known as regression and transgression, have occurred. As a result, space has been created for the deposition of sediments carried by the Pearl River. The Quaternary geological sequence, which overlays the bedrock, exemplifies this cycle. It comprises marine units, primarily constituted by silt and clay, as well as continental units, dominated by sand and gravel. The bedrock underlying the delta exhibits complexity due to varying deformation and intrusion events prompted by significant tectonic activities. The ongoing evolution of the delta is principally influenced by neo-tectonic movements, fluvial processes, fluctuations in sea level, and human activities. These elements collectively exert a significant impact on the dynamic development and transformation of the delta.

Geomorphology
The Pearl River Delta, typically flat and low-lying, has a maximum elevation of 10 meters above sea level, a characteristic feature of deltaic formations. The delta's formation is largely attributed to the deposition of sediment transported by its three main sub-rivers: the East River, the North River, and Xi Jiang, which collectively flow into the Pearl River Estuary. Despite its predominantly low relief, the delta plain, which spans approximately 9750 km², is interspersed with over 160 islands in varied forms such as hills and monadnocks, with elevations ranging from 100 to 300 meters. These rocky islands comprise about one fifth of the delta's total area (~2000 km²). This unique geomorphological feature is the result of a combination of fault block activities and diverse bedrock.

The Pearl River Delta represents a complex deltaic system. Unlike the Mississippi River Delta, a textbook example of river delta, it does not neatly fit into William Galloway’s triangular delta classification, which categorizes deltas into three types: river-dominated, wave-dominated, and tide-dominated. The Pearl River Delta is influenced both by tidal and wave processes, with certain parts dominated by fluvial processes. Due to the presence of bedrock islands in the estuary area and the main river being constrained by the steep relief surrounding the deltaic region, the main channel's course has remained relatively stable, preventing the formation of deltaic lobes akin to those in the Mississippi River Delta. Consequently, the deltaic plains are concentrated in the current delta area.

Geological setting
The Pearl River Delta is situated on the southern edge of the Cathaysia Block, one of the key crustal blocks in southeastern China. This region is situated on the Eurasian tectonic plate, abutting the oceanic crust along the margins of the South China Sea. While it is located at the edge of a passive continental margin, the delta experiences a primary tectonic stress field in the north–south direction due to the compressional motion of the Indian Ocean Plate and the Philippine Sea Plate from various directions.

The stress field within the Pearl River Delta stimulates the activity of the area's faults, considerably influencing the delta's evolution. The consequent movement and behavior of these faults are key determinants in the continuous development and transformation of this geologically dynamic region. As a result, the behavior of faults and the differential movements of associated fault blocks primarily dictate the neotectonic features in this region, where substantial seismic activity is an occasional phenomenon.

Geological structure


The geological structures of the Pearl River Delta are primarily fault-dominated. Fold structures are relatively scarce in this region, with the nearest large-scale fold structure, the Nanling-Wuyi fold belt, situated significantly distant from the delta.

The fault systems in the deltaic region comprise three main sets, including normal, reverse, and strike-slip faults. These faults, which emerged in varying time periods, are categorized based on their trending directions: NE-SW, NW-SE, and W-E. Many of these faults have significantly contributed to the local geomorphological development. Particularly, the first set of faults, which largely shape the morphology of the coastline and the Pearl River Delta. The Shenzhen fault is a key lineament following this trend.

The interaction of these three sets of faults has resulted in the formation of multiple fault blocks that have experienced differential subsidence. These formed fault blocks are crucial in determining the delta's evolution. The rate of sedimentation and the development of river channels in the delta are greatly influenced by the subsidence rate and spatial distribution of these fault blocks. Increased subsidence rates often lead to an enhanced sedimentation rate, resulting in thicker sediment deposition. Additionally, fault zones' susceptibility to weathering and erosion has created weak zones in the basement topography, guiding the development of the main river channels in the delta. The accommodation space for Quaternary deposits was predominantly created by such neotectonic activity.

To illustrate the importance of such fault sets, seven fault blocks created by thirteen faults around the main delta are selected. They include:

Stratigraphy
The bedrock in the Pearl River Delta consist of some very old rock dated back to Precambrian and are complex in nature. This is product of metamorphism and magmatic intrusion experienced by the sedimentary bedrock in the Pearl River Delta caused by several major tectonic events. The stratigraphic sequence in Pearl River Delta mainly consisted of a Neo-Proterozoic meta-sedimentary basement unit with a overlying discontinuous rock records from Paleozoic to Mesozoic composed of terrigenous sedimentary rock interbedded with some carbonate unit. The volcanic and intrusive events occurred during the Upper Mesozoic to Tertiary formed granites intruded in these layers of rock and, tuff formed on top of them. At the top, Quaternary deposits consisted of two continental units and two marine units filled the depressed area in the region. This complex stratigraphy is grouped into nine rock units:

Geological history
The Geological history of Pearl River Delta spans across 600 million years and can be divided into five main periods: the pre-Cretaceous period, the Cretaceous period, the Tertiary period, the Quaternary period, and the recent Holocene epoch.

Paleozoic and Neoproterozoic Era
In the Ediacaran period, approximately 600 million years ago, the region of the Pearl River Delta was characterized by a structurally depressed area within a deep marine environment. It is theorized that this region constituted an oceanic basin during that period, fostering a depositional environment that facilitated the formation of bioclastic and clastic sedimentary rock. In the Paleozoic eon, around 540 million years ago, the depositional environment underwent significant changes. The sedimentary facies formed during this period were markedly different from the Ediacaran bedrock, including notable examples such as shallow marine facies and terrigenous facies.

Mesozoic Era
The Mesozoic era, which began approximately 252 million years ago, was characterized by a primarily lacustrine environment consisting of terrestrial shale interbedded with sandstone. This sedimentation process persisted until substantial tectonic activities ushered in multiple phases of faulting and magmatic intrusion. From the Jurassic to the Cretaceous period, the westward subducting paleo-Pacific Plate triggered the Yanshanian movement which led to the intrusion of granitic magma. The regional fault zones guided the intrusive motion of the magma, leading to the formation of the Mesozoic Magmatic Belt in the SE China from the existed large scale NE-SW fault zones. The granites and tuff, which are widely distributed in Hong Kong and the southeastern and eastern areas of the Pearl River Delta, are representative of this magmatic belt. These tectonic and magmatic events induced the deformation of the Ediacaran bedrock through contact and regional metamorphism, transforming it into metasedimentary rock. This period also saw the rapid development of fault block mountains and fault basins. The effects of weathering and erosion led to the formation of hills and mountains from the fault blocks. The weathered material from these mountains was transported and deposited in the fault basins. Consequently, the evolving basin preserved the records of marine transgression and volcanic activity by retaining the land and marine strata from the late Mesozoic to the early Tertiary period. The Himalayan movement, initiated by the collision of India with Asia since the middle Tertiary, uplifted the delta basin through rapid crustal modifications. This uplift facilitated erosion and incision, but the eroded material was not deposited in the Pearl River Delta. Instead, it was transported more than 200 km away from the delta area.

Cenozoic Era
Throughout the Tertiary and Quaternary periods, the uplift of the Tibetan Plateau significantly altered the topography, which in turn modified the drainage pattern and led to the formation of the Pearl River drainage basin. This geological event also initiated the rifting of several delta basins due to the movement of fault blocks, resulting in an overall subsidence of the basin adjacent to the estuary in the Late Quaternary. The subsidence of the deltaic region during the Late Quaternary created ample accommodation space, prompting a new phase of deposition and halting the bypassing of sediment to the adjacent continental shelf in the South China Sea. The interplay of repeated sea-level changes and subsidence from fault block movements fostered the formation of two deltas at different times. As a result, two stratigraphic sequences were formed, each featuring a combination of terrestrial and marine sedimentary layers. These sequences rest atop sedimentary rocks that date back to the Cenozoic and Mesozoic periods, and are also underlain by Mesozoic igneous rocks.


 * The older sequence of the deltaic deposits traces back to the first cycle of marine regression and transgression during the last interglacial period, which occurred between 130,000 and 115,000 years ago.
 * The more recent sequence of the deltaic deposits pertains to the second cycle of marine regression and transgression. This cycle has been ongoing during the current interglacial period, which spans from 11,500 years ago to the present day.

Evolutionary model in the Holocene
The modern configuration of the Pearl River Delta emerged during the second marine transgression, which took place approximately 7500 years ago. To elucidate the factors driving the evolution of the present delta, a three-stage evolutionary model has been proposed.

Stage 1 (9000–6800 years ago)
The swift environmental changes during the early Holocene were primarily driven by a rapid increase in sea level and, secondarily, by strong monsoon runoff. The interaction of these two factors resulted in the flooding of the deltaic basin by seawater, which led to a shift in sedimentation facies from fine sand to silt and clay. This stage was characterized by a transition from shallow to deep tidal processes in the receiving basin. Subsequently, a transgression phase commenced around 8000 years ago, which after approximately 1200 years (around 6800 years ago) transitioned into a regression phase.

Stage 2 (6800-2000 years ago)
During the subsequent stage, tides and monsoonal discharge became the dominant mechanisms as the sea level began to stabilize. These factors spurred the delta's growth until approximately 6000 years ago, a process driven by the continuous weakening of the summer monsoon. From this point onwards, the delta front facies transitioned to become the primary sedimentary facies, which allowed for the modification of sediments by tidal action.

Stages 3 (2000 years ago-present)
As the impact of the monsoonal discharge continued to weaken and human activities increased, these human activities became a significant factor during this stage. Despite an increase in sediment supply due to deforestation, most sediments were trapped in reclaimed land or tidal flats. A decrease in the rate of vertical accretion was noted, likely due to a reduction in the amount of sediment reaching the estuary. This could be attributed to a rapid advancement of the shoreline caused by swift land reclamation.

This model reveals a continuous advancement of the Pearl River Delta's shoreline, a trend driven by increased human activities and decreased influences of natural factors such as sea-level changes and monsoonal discharges.

Current geological issues
The Pearl River Delta faces two significant geological issues: pollution and subsidence.

Water pollution
The primary sources of contamination are chemical spillages, unauthorized factory discharges, and urban water-logging. These human activities significantly impact the dynamics of groundwater, rendering the confined aquifer increasingly susceptible to pollution. As a result, it is advised to abstain from exploiting deep confined aquifers if shallow aquifers are already polluted. To safeguard the crucial groundwater resources around the Pearl River Delta, implementing best practices such as the construction of more efficient water treatment plants and irrigation with clean water are recommended.

Land subsidence
Subsidence represents the second significant issue experienced in the Pearl River Delta. Although surface subsidence is a natural part of the geological evolution of the area, human activities within the deltaic region have accelerated this process. The extent of this acceleration can be quantified through InSAR (Interferometric Synthetic Aperture Radar), a remote sensing technology. According to InSAR data, an average subsidence velocity of 5 cm/year was observed in some regions, a rate significantly higher than those reported in earlier studies.