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 overlies 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 the 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 results from a combination of fault-block activity and diverse bedrock.
The Pearl River Delta represents a complex deltaic system. Unlike the Mississippi River Delta, a textbook example of a 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 by both 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 subsequent movement and behavior of these faults are key determinants of 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: normal, reverse, and strike-slip faults. These faults, which emerged at different times, are categorized by 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 largely shapes 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 fault sets 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 enhanced sedimentation rates, 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: +FaultFault typeFault setsHistoryFault BlockNote1. Sanshui–Luofushan FaultNormal faultW-EThe Yanshanian Movement in the Cretaceous period was responsible for the formation of the faults. These structures underwent several periods of reactivation in subsequent geological processes.N/AN/A2. Guangzhou–Conghua FaultNormal faultNE-SW(I) Dongjiang Delta fault block, (II) Tan Hoi fault block, (III) Wuguishan fault block, (IV) Shiqiao–Guangzhou fault block(I) and (VII) South China Sea fault block Strong activities of the fault block Center of deposition Well preserved in the rock records and can be seen from field evidence.3. Dongguan–Houjie FaultStrike-slip fault4. Shiqiao-Xinhui FaultStrike-slip fault5. Wuguishan FaultStrike-slip fault6. Shenzhen–Zhuhai FaultStrike-slip fault7. Pearl River Estuary FaultStrike-slip fault8. Xijiang FaultNormal faultNW-SEThe Himalayan Movement, which took place during the Oligocene-Pliocene epochs, was instrumental in the formation of these fault structures.(V) Xijiang–Beijiang Delta fault block and (VI) Pearl River estuary fault block and Weak activities of the fault block9. Baini–Shawan FaultReverse fault10. Nangang–Humen FaultStrike-slip fault11. Hualong–Huangge FaultNormal fault12. Yamen FaultStrike-slip fault
Stratigraphy
The bedrock in the Pearl River Delta consists of very old rock dating back to the Precambrian and is complex in nature. This is a 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 an overlying discontinuous rock record from Paleozoic to Mesozoic, composed of terrigenous sedimentary rock interbedded with some carbonate units. The volcanic and intrusive events that occurred during the Upper Mesozoic to Tertiary formed granites that intruded into these rock layers, 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: +Rock units in Pearl River DeltaPeriodsUnit nameDominant rock typesEnvironmentNoteQuaternary1. Quaternary unitLoose deposits with a wide range of clasts, but with a majority of sand and silty sand DeltaLength of the bed is 20-60mCretaceous to Tertiary2. Mesozoic to Tertiary reworked tuff and alluvial unitReworked tuff breccia and conglomerate, interbedded with sandstone and shaleLake, river, and alluvial fanA sharp angular unconformity and red bed sedimentation Jurassic to Cretaceous 3. Upper Mesozoic volcanic unitRhyo-porphyry, dacite Porphyry, quartz conglomerate, volcanic breccia, interbedded with layers of tuff, sandstone, shale, and siltstoneVolcanic arcAsh tuff in Hong KongJurassic to Cretaceous 4. Upper Mesozoic intrusion unitGranite, diorite, and monzoniteVolcanic arcGranite and rhyolite of calc-alkaline to high-K calc-alkaline series, the Luofushan plutonTriassic to Jurassic 5. Mesozoic terrigenous unitSiltstone and shale are interbedded with a sandstone layerLakeN/ACarboniferous to Permian6. Paleozoic terrigenous unitQuartz sandstone with siltstone, marls, fine sandstone, and conglomerateLake, river, and alluvial fanN/ACarboniferous to Permian7. Paleozoic carbonate unitLimestone, dolostone, and carbonate marlsShallow marineN/ADevonian 8. Paleozoic terrigenous clastic unitConglomerate, quartz conglomerate, quartz sandstone, and quartiteLake, river, and alluvial fanN/ASilurian to Ediacaran9. Basement unitSlate, siliceous shales, graptolites shales, and graptolite shales with a layer of sandstone. And metamorphic facies such as phyllite, schist, and gneissDeep marineN/A
Geological history The Geological history of the 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