Fluidal Peperites Recorded in the Cretaceous Lacustrine Sediments in the Southern Korean Peninsula: Syn-Magmatic Sediment Fluidization and Its Influence on the Peperite Formation

Abstract

This study assessed the influence of sediment and water redistribution in host sediments on peperite formation by examining the peperites at the boundary between Cretaceous lacustrine sedimentary successions and intruding dikes (D1 and D2). The peperite zones occur along the dike margins and consist of fluidal juvenile fragments, classified as Type A and Type B perperite zones based on lateral extent of the peperite zones. Type A peperite zone, the dominant type, exhibites a narrow distribution (<20 cm), whereas Type B peperite zone sporadically occurs along D1 with a wider width (<1 m). Type B peperite zone is laterally linked with clastic dikes. These dikes containi fluidal shaped dike fragments with jigsaw-fit textures, indicating syn-magmatic fluidization and the resultant formation of the clastic dike via heat transfer. During dike emplacement, the interaction between the host sediments and the intruding magma formed Type A along the margins. Simultaneously, the clastic dikes, composed of fluidized sediments and water, supplied additional water and sediments, enhancing magma-host sediment intermingling and leading to the wide lateral extent of Type B. Our findings demonstrate that sediment and water redistribution via syn-magmatic fluidization is crucial in peperite formation, influencing the initial processes of phreatomagmatic volcanism.

1. Introduction

Peperites are rock records of interactions between molten magma or lava and poorly consolidated or unconsolidated, typically wet sediments (host sediments) [1,2,3]. Due to the unique phenomena and conditions involved in their formation, peperites provide evidence of the timing and types of syndepositional volcanism, allowing for a detailed reconstruction of basin evolution. Additionally, peperites capture the initial processes of fuel-coolant interactions in volcanic vents. Detailed observations and analysis of peperite-forming processes improve our understanding of phreatomagmatic volcanic eruptions, such as Surtseyan-type eruptions [4,5]. Peperite-forming processes involve the fluidization of nearby wet sediments due to heat transfer and associated volume expansion via phase changes of pore water to vapor [3]. A concomitant increase in pore water pressure causes the sediment to become fluidized by sediment grains supported by upward-moving fluids. When the increased pore water pressure exceeds the overburden pressure, mixtures of water and sediment are typically injected into the overlying, non-fluidized beds, forming clastic dikes [6]. This results in the redistribution of water and sediments in the host sediments. Because the occurrence and morphological characteristics of peperites are known to be controlled by the water volume and physical characteristics of the host sediments [7,8,9,10,11,12], it is important to evaluate the influence of the fluidization of the host sediment on peperite-forming processes.

This study investigates the Cretaceous lacustrine sedimentary successions exposed in coastal rock cliffs and wave-cut platforms in the southern Korean Peninsula. In the study area, the lacustrine sedimentary successions are intruded by magmatic and clastic dikes. Peperite zones commonly develop at the interface between the host sediments and dikes and are mostly composed of fluidal shaped juvenile fragments. However, lateral extent of the peperites zones are markedly different. The peperites zones commonly exhibit narrow distribution (Type A peperite zone, <20 cm in width). On the other hand, the Type B peperite zones are wider lateral extent (<1 m in width) and laterally linked with clastic dikes. The clatic dikes include fluidal-shaped dike fragments showing jigsaw-fit texture, implying syn-magmatic fluidization and associated redistribution of water and sediments via clastic dikes. This study aims to describe and interpret the occurrence and morphological characteristics of juvenile fragments in peperite zones and discuss the influence of syn-magmatic sediment fluidization on peperite-forming processes.

2. Geological Background

During the Cretaceous, the eastern part of the Eurasian Plate, including the Korean Peninsula, experienced continental arc volcanism and crustal deformation due to the subduction of the paleo-Pacific Plate (Izanagi Plate) underneath the Eurasian Plate [13,14]. This crustal deformation created a series of non-marine sedimentary basins in the eastern part of the Eurasian Plate, where sedimentary strata are commonly interbedded with volcanic rocks [15]. The Cretaceous Gyeongsang Basin is one of the nonmarine sedimentary basins, located in the southeastern part of the Korean Peninsula (Figure 1) [16,17]. The basin fills are represented by the Gyeongsang Supergroup, which unconformably overlies the Yeongnam Massif composed of Paleoproterozoic metamorphic rocks and Triassic and Jurassic granitic rocks [18]. Sedimentological, stratigraphical, and paleontological studies suggest that the Gyeongsang Supergroup was deposited in fluvio-lacustrine environments, mostly under warm and arid climatic conditions. Based on the abundance of volcaniclastic sediments and interbedded volcanic rocks within the groups, the Gyeongsang Supergroup is subdivided into three stratigraphic units from bottom to top: the Sindong, Hayang, and Yucheon groups [16,18,19,20,21,22]. The lowermost Sindong Group accumulated in the Nakdong Trough, which developed close to the Yeongnam Massif (Figure 1). This group is composed of terrigenous clastic sediments derived from the Yeongnam Massif, although volcanic rock fragments and volcanic quartz are rarely found [23]. Detrital zircon analysis suggests that this group began to be deposited after 127 Ma [24]. After the deposition of the Sindong Group, the Gyeongsang Basin was subdivided into the Jinju (also known as Miryang), Euiseong, and Yeongyang subbasins, bounded by intrabasinal faults and highs (Figure 1) [21]. In the Jinju subbasin, where the study area is located, the overlying Hayang Group is composed of fluvio-lacustrine sediments interbedded with basaltic lava flows and basin-wide tuff beds (Gusandong Tuff) [25]. The sediments in this group also contain large amounts of volcanic rock fragments as well as volcanic-origin crystals, indicating syndepositional volcanic activities [17]. From bottom to top, this group is classified into the Chilgok Formation, Silla Conglomerates, Haman Formation, and Jindong Formation. The uppermost Jindong Formation was deposited in shallow lacustrine environments, while the others were deposited in fluvial environments. Detrital zircon analysis suggests that the Hayang Group began to accumulate after 108 Ma [24]. The Hayang Group is overlain by the topmost Yucheon Group. This Group represents a climactic period of volcanic activities during the deposition of the Gyeongsang Supergroup. It is composed of widespread andesitic to rhyolitic volcanic rocks interbedded with tuffaceous sediments, resulting from basin-wide explosive volcanic eruptions (Figure 1) [16]. Field examination and geochemical analysis suggest that these volcanic activities are genetically related to caldera volcanism in a continental volcanic arc [16]. Sensitive high-resolution ion microprobe (SHRIMP) and laser ablation multi-collector inductively coupled plasma-mass spectrometry (LA–MC–ICPMS) U-Pb zircon analyses suggest that these volcanic activities persisted from 97 to 96 Ma to the Late Paleogene [26,27].

The Jindong Formation, where the studied peperites occur, is an approximately 1500 m thick sedimentary successions. This formation is characterized by laterally continuous mudstones and sandstones, locally interbedded with limestones and tuff beds. Symmetric ripples are common on the bedding planes, and post-depositional modifications, such as dinosaur footprints, raindrop impressions, and desiccation cracks, are well developed [20,28]. Additionally, these mudstones and sandstones commonly contain calcrete nodules ranging from centimeters to decimeters in length [16]. Based on these depositional features, the Jindong Formation is interpreted to have been deposited in marginal lacustrine environments under arid climatic conditions, where paleo-lake levels frequently fluctuated [28].

3. Methods

We examined the host sediments to determine grain size, bed geometry, lateral continuity, and post-depositional modifications. The dikes intruding into the host sediments were also measured to identify their outcrop occurrence (e.g., orientation, width, and lateral variations) and texture. The occurrence of peperites developed along the boundary between host sediments and dikes was analyzed to determine their width and lateral variations. The juvenile fragments in the peperite zones were classified according to the schemes suggested by Busby-Spera and White [1] and Skilling et al. [3]. Afterward, the dikes and juvenile fragments in the peperites were sampled for thin section analysis and major element whole-rock analysis using X-ray fluorescence (MXF-2400, Shimadzu, Kyoto, Japan) installed at Korea Institute of Geosicence and Mineral Resources (KIGAM), Deajeon, South Korea to determine the composition between dikes and juvenile fragments.

4. Results

4.1. Host Sediments

In the study area, the host sediments occur in coastal rock cliffs (>20 m) and on basal wave-cut platforms (Figure 2 and Figure 3). The host sediments are mostly composed of laterally continuous (<300 m) mudstone and sandstones couplets, gently dipping (<10°) southeastward (<10°). The mudstone layers of the couplets are grey to pale grey in color and internally massive (Figure 4a). These mudstone layers range in thickness from mm to 20 cm. The sandstone layers of the couplets are composed of fine to very fine sand and are less than 5 cm thick. The sandstone layer is also internally massive, with normal grading locally found at the top of the layer. These couplets are interbedded with normally graded sandstone beds. These sandstone beds are less than 30 cm thick with sharp lower boundaries. Crude planar stratifications are locally observed in the upper part of the beds. On the bedding planes of these host sediments, symmetric ripples and desiccation cracks are common, and dinosaur footprints also occur frequently (Figure 4b,c). The host sediments, particularly in mudstone and sandstone couplets, contain large amounts of elliptical-shaped calcrete nodules ranging from 5 cm to 30 cm in their long axis (Figure 4d).

The laterally traceable, fine-grained sediments suggest that the couplets of mudstones and sandstones were deposited by the settling of suspended sediments in low-energy conditions [30]. Symmetric ripples on the bedding plane reflect wave modification after deposition, indicating that the host sediments were deposited in marginal lacustrine environments subjected to oscillatory bottom waters [30]. The normally graded sandstone beds with crude planar stratifications indicate deposition from decelerating turbulent flows, likely formed by the partial collapse of sediments deposited in marginal lakes [31]. The common occurrence of desiccation cracks and dinosaur footprints suggests fluctuations in the water level of the lake, likely due to episodic flooding and subsequent shrinkage under arid climatic conditions [28]. This interpretation is consistent with the common occurrence of calcrete nodules in mudstone and sandstone couplets, formed as a result of the precipitation and growth of oversaturated calcium carbonate in pore spaces during shrinkage [32].

4.2. Magmatic Dikes

On the wave-cut platform, the host sediments are intruded by two dikes (D1 and D2). D1 is up to 2.5 m wide and can be traced laterally to the ends of the platform. In contrast, D2 is less than 30 cm wide, tapering laterally and eventually disappearing (Figure 5a). At the end of D2, the bedding of the host sediments is disrupted by water-escape structures (Figure 5a). The distance between the two dikes is an approximately 3 m. Both dikes are aligned N5°E (D1) and N15°E (D2) subvertically and are laterally contact with the peperite zones with irregular boundaries. The dikes are greenish-grey or pale-grey in color, and at the microscopic scale, euhedral to subhedral plagioclase crystals are surrounded by a microcrystalline groundmass, exhibiting porphyritic textures (Figure 5b). Both dikes are composed of coherent central bodies surrounded by flow-banded lateral margins (Figure 5c). Centimeters-scale, elliptical-shaped cavities occur at the margins of D1, with their long axes also parallel to the dike margin (Figure 5c). These cavities are commonly empty; however, locally filled with calcite. Locally, the dikes are invaded by matrix of peperites (homogeneous sandy mud, see below) (Figure 5d). Based on major element analysis, these dikes are trachyandesite in composition (

Table 1. Whole rock major-oxides (wt.%) of dikes and juvenile fragments of peperites.

Sample Locations	SiO2	Al2O3	Fe2O3	CaO	MgO	K2O	Na2O	TiO2	MnO	P2O5	LOI*	Total	Remarks
230601-A	54.23	16.06	9.44	4.71	3.31	1.68	4.24	0.85	0.14	0.15	4.43	99.24	Dike-1
230601-B	56.96	14.94	8.78	3.12	3.04	1.37	5.78	0.77	0.18	0.16	4.41	99.51	Dike-1
230601-F	53.22	17.90	8.44	2.14	3.42	1.98	5.81	0.77	0.12	0.20	5.31	99.31	Juvenile fragment
230601-G	50.96	16.15	7.83	5.96	3.61	0.86	6.28	0.67	0.15	0.18	6.56	99.21	Juvenile fragment
230601-H	52.49	18.12	8.17	3.58	3.21	2.45	5.18	0.78	0.13	0.17	4.81	99.09	Dike-1
230601-I	53.87	18.31	6.41	3.91	2.82	2.50	5.55	0.76	0.07	0.17	4.53	98.90	Dike-2

LOI*: Loss on ignition.

; Figure 6).

4.3. Clastic Dikes

Clastic dikes occur between D1 and D2 (Figure 7a). These dikes are less than 30 cm in width and has irregular, curved lateral margins. These dikes consist of the brecciated sediments set in the matrix composed of homogeneous sandy mud. These brecciated sediments are pebble to cobble in size (up to 20 cm long) and have sharp or curvi-linear edges, showing angular to subangular shapes (Figure 7b,c). Internally, these sediments are composed of alternating layers of mm to cm thick, mud and sand (Figure 7c), exhibiting identical features to those of the mudstone and sandstone couplets in the host sediments (brecciated host sediments). The clastic dikes also contain fragments of the magmatic dikes intruding the host sediments. The dike fragments amoeboidal or elongated globular shapes, defined by irregular, curved margins (Figure 7c). Locally, these dike fragments show a jigsaw-fit texture (Figure 7c). In the study area, the clastic dikes are 0.3 to 3 m in length, and one of the clastic dikes are linked with Type B peperite zones.

4.4. Peperite Zones

Peperite zones are developed along the margins of the dikes. They are defined by dispersed or closely packed dike fragments (juvenile fragments) set in the homogeneous sandy mud matrix. Primary sedimentary structures (e.g., bedding) of the matrix are completely disrupted. Away from the dike margins, the peperite zones are laterally gradational to the host sediments where bedding and lamina are well preserved. Major element analysis revealed that the juvenile fragments are also trachyandesite in composition (Table 1; Figure 7).

Based on their occurrence and lateral extent of the peperite zones, the peperites are classified into Type A and Type B peperite zones. Type A peperite zone occurs along both dike margins and is less than 20 cm in width (Figure 8a). In this zone, the juvenile fragments are closely packed along the dike margins and become more dispersed as the abundance of the juvenile fragments decreases with distance from the margin (Figure 8a). The juvenile fragments range in size from coarse ash to block (up to 15 cm in length), but lapilli-sized juvenile fragments are common. These fragments have irregular or curved margins, exhibiting amoeboid or elongated globular shapes (Figure 8b), and are categorized as fluidal peperites [3].

Type B peperite zone is sporadically developed only along D1, and its lateral extent is a wider (less than 1 m) than that of Type A peperite zone (Figure 8c). Type B peperite zone is also composed of fluidal-shaped juvenile fragments set in the matrix composed of homogeneous sandy mud. With the juvenile fragments, the fragmented sediments composed of multiple lamina of mud and sand are common. These fragmented sediments are pebble to cobble in size and are angular to subangular in shapes, exhibiting similar features of the brecciated host sediments in the clastic dike (Figure 8d). Indeed, in the studied outcrop, Type B peperite zone is laterally linked with the clastic dike (Figure 5a).

4.5. Formative Processes of Peperites and Clastic Dikes

The presence of the homogeneous sandy mud matrix of the perpite zones in the dikes and the disruption of primary sedimentary structures suggest fluidization of the host sediments [1,3,34,35], indicating an unconsolidated and wet state of host sediments during magma emplacement. Heat transfer from the magma to the host sediments resulted in an increase in pore water pressure and associated volume expansion with the detachment of sediment grains [11], leading to a fluid-like state of the host sediments. In addition, the increased pore water pressure enabled to have muddled the well-stratified host sediments with destruction of original sedimentary structures, resulting in the formation of homogeneous sandy mud matrix in peperite zones. The modified physical conditions of the host sediments allowed intermingling with the magma via fluid-fluid instability (e.g., Kelvin-Helmholtz or Rayleigh-Taylor instability), causing lumps of magma to detach from the main magma bodies [34]. In this scenario, the fine-grained and low-permeability of the host sediments provided favorable conditions for the development of a vapor film on the detached magma and dike margins [4,36]. The vapor film acted as an insulator, allowing the detached magma to deform in a ductile fashion, resulting in the common occurrence of fluidal-shaped juvenile fragments rather than blocky ones.

Clastic dikes are products of sediment fluidization and commonly form by breaking through and subsequently injecting into the overlying sediments due to elevated pore water pressure [37,38]. Fluidization experiments demonstrated that elevated pressure in water-saturated sediments detaches sediment grains, which are then supported by upward-moving waters, forming a fluidized layer [8,38,39,40,41]. Upward-moving water and sediment mixtures tend to sink if they encounter an overlying, non-fluidized layer, forming a convective cell. The experiments showed the development of multiple convective cells in a single fluidized layer [41]. When elevated pore water pressure exceeds the overburden pressure, the water and sediment mixtures tear apart and intrude the overlying layers at the top of each convective cell, resulting in multiple intrusions and the formation of finger-like clastic dikes [37,39,40,41]. The broken fragments of the overlying non-fluidized layers are commonly entrained by the upward-moving water and sediments probably with preservation of original sedimentary structures. In the study area, the homogeneous sandy mud in the clastic dikes was probably formed as a result of mixing of the host sediments via the convective cells within fluidized layers and/or entrainments of sediments during the intrusion. In addition, a number of brecciated host sediments are interpreted to have been formed by breakage of the host sediments and subsequent entrainment during the intrusion of clastic dikes.

5. Discussion

The common occurrence of fluidal juvenile fragments in the study area suggests that homogeneous sandy mud derived from fluidization of the host sediments provided favorable conditions for the ductile deformation of the intruding magma. Although the morphological characteristics of juvenile fragments are identical, the differing lateral extent between Type A and Type B peperite zones are suggestive of spatial variations in physical conditions of the host sediments during peperite formation. Hanson and Wilson [35] studied rhyolite peperites formed as a result of interaction between multiply intruding rhyolitic magmas and wet host sediments composed of mudstone and chert interbedded with sediment gravity flows deposited in submarine environments below storm wave base. Despite the large-scale (3 km² wide and 300 m high) development of peperites along the margins of intrusion, coherent rhyolites without peperites also occurred. The authors suggested that these differences reflected contrasting physical conditions for peperite formation, which were attributed to variations in (1) magma viscosity and supply rates, (2) degree of consolidation of the host sediments, and (3) volume of pore waters. In the study area, variations in magma viscosity and supply rates are not applicable, as both D1 and D2 are composed of a single set of central coherent bodies and flow-banded rims, indicating a single intrusion event. The degree of consolidation largely depends on lithology and burial depth. However, all juvenile fragments are set in homogeneous sandy mud formed by mixing of host sediments via fluidization, and uniform lithology of the host sediments (mudstone and sandstone couplets and normally graded sanstones) indicates no or minor changes in local lithology. Additionally, since the peperite zones occurs in a single, studied outcrops (ca. 20 m thick) differences in the burial depth of the host sediments did not play a critical role in the degree of consolidation, as suggested by Galletly et al., 2024 [42].

On the other hand, large amounts of brecciated host sediments in Type B peperite zones and their lateral connection of with clastic dikes would provide clues for the spatial variations. Although dike fragments in the clastic dike appear to indicate later intrusion of the clastic dikes after magmatic dike emplacement (D1 and D2), the fluidal-shaped dike fragments in the clastic dikes suggest ductile deformation of molten magma and their jigsaw-fit textures point out heat retention after entrainment by clastic dikes (Figure 5) [1,2,3]. This reflects a short time interval between magma emplacement and clastic dike formation, implying that clastic dikes were formed by syn-magmatic fluidization as a result of heat transfer to surrounding sediments, as reflected by the water-escape structures in the host sediments occurring at the end of D2 (Figure 5) in the study area. Heat transfer from magma to the surrounding sediments resulted in a phase change of pore water to vapor with volume expansion. The resulting increase in pore water pressure caused upward-moving mixtures of water and sediment, forming clastic dikes composed of homogeneous sandy mud (Figure 9). Large amounts of brecciated host sediments in the clastic dikes result from the disintegration and subsequent entrainment of the brecciated host sediments during the intrusion (Figure 9) [43]. As the clastic dikes intruded host sediments and reached the dike margins where juvenile fragments were formed, the mixtures provided additional water and fine-grained sediments because the clasitc dikes inherently contain large amounts of water. This may provide favorable conditions for the formation of a vapor film and, hence, intermingling of magma with surrounding sediments [4,34], resulting in the wide development of Type B peperite zones along the margin of Dike 1 (Figure 9). Additionally, the sporadic occurrence of Type B peperite zones is attributed to the finger-like intrusion of clastic dikes, as demonstrated by fluidization experiments [37,39,40,41].

6. Conclusions

The present study demonstrated that the combination of syndepositional trachyandesite magmatism and fine-grained host sediments resulted in the development of fluidal peperites along the dike margins (both Type A and Type B peperite zone). Type A peperitezone shows a narrow distribution (width: <0.2 m), whereas Type B peperite zone is wider (width: <1 m) and connected with the clastic dikes. These differences are interpreted to be as a result of contrasting physical conditions along the dike margins during the peperite formation. Syn-magmatic sediment fluidization as a result of heat transfer from magma caused the development of clastic dikes with redistribution of water and sediments. The additional supply of water and fine-grained sediments via clastic dikes to the dike margin created favorable conditions for the intermingling of host sediments and magma, resulting in the wide development of peperites (Type B peperite zones). Thus, the heat transfer from magma and associated fluidization caused spatial variations in the physical conditions of the host sediments via redistribution of pore water, influencing the peperite-forming processes.