1. Introduction

Shallow marine structures are sediment deformations and ruptures developed in a shallow environment less than 1000 m below the seafloor (mbsf) due to many kinds of driving forces such as overpressure, differential gravity, compaction forces, tectonic forces, etc. Berton and Vesely [1] identified comet pockmarks on acoustic profiles and suggested that they are formed by gas flows and modified by the bottom currents. Shakhova et al. [2] detected pockmarks using multi-beam sonar backscatter data and proposed that gas release causes the pockmarks. Chen et al. [3] identified gas chimneys on seismic profiles and indicated that they are related to thermogenic gas migration. Matsumoto et al. [4] identified gas pipes on seismic profiles and suggested that they are induced by thermogenic gas migration. Chen et al. [5] identified mud diapirs and mud volcanoes on seismic profiles and indicated that their formation mechanisms are related to an ample discharge of gas-bearing fluids. Ho et al. [6] studied the morphology of pipe structures and polygonal faults and suggested that they are formed by overpressured gases and fluids. Previous studies show that shallow marine structures are generally considered as pathways for gas and fluid migration. In recent years, the structures related to gas hydrate accumulation have increasingly attracted researchers’ attention.

Gas hydrates have been intensively studied as a potential clean energy source in the last decades ^[7][8][9][7,8,9]. They are stable under relatively low temperature and high pressure conditions, and are generally located in sediments several hundred meters deep or even deeper below the seafloor ^{[10][11][12][13]}[10,11,12,13]. Massive gas hydrate dissociation can release vast amounts of methane into seawater and enhance ocean acidification, which is a significant process affecting the carbon balance in the marine environment ^[14][15][16][14,15,16]. The accumulation and distribution of gas hydrates are closely related to the gas and fluid migration which is primarily controlled by the structures in shallow marine sediments ^{[17][18][19][20][21]}[17,18,19,20,21]. Wang et al. [22] studied three cold seeps in the Taixinan Basin and found massive gas hydrate and chimney structures associated with these cold seeps. Lu et al. [23] studied a mature pockmark in the Zhongjiannan Basin and indicated that the pockmark creation was induced by methane consumption. Natalia and Soledad [24] recognized gassy sediments, pockmarks and gas plumes in the Ria de Vigo in Spain and indicated that the underlying gassy sediments are the gas sources of the pockmarks. Berton and Vesely [1] identified mud volcanoes and underlying chimney structures on the continental margin of southeastern Brazil.They suggested that gas seeps occur along with the chimney structures. Taylor et al. [25] recognized salt diapirs and associated faults at the Blake Ridge Diapir, indicating that the gas and fluid might migrate upward through these faults. Lastras et al. [26] recognized shallow slides and pockmarks in the Eivissa Channel and suggested that the pockmarks occur near the slide headwall scars that reveal fluid escaping. Alrefaee et al. [27] studied polygonal faults and associated fluid migration in the Northern Carnarvon Basin and indicated that the polygonal fault systems can act as pathways for the fluid migration. Gasperini et al. [28] studied the tectonic fault in the Sea of Marmara and suggested that gas and fluid emissions increase along the tectonic fault system after an earthquake.

2. The Classification of the Structures

The structures are generally driven by different geological forces, including overpressure, gravity, compaction forces and tectonic forces.

Overpressure is widely developed in many hydrocarbon basins, and its building, transfer, and release generally induce different structures. A seepage occurs during the overpressure release (Figure 1A), a gas chimney develops during the overpressure building and transfer (Figure 1C), and a pockmark is a seafloor mark of the overpressure release site (Figure 1B). Therefore, seepages, pockmarks and gas chimneys are classified as overpressure-associated structures. Seepage may be referred to as a natural phenomenon rather than a geological structure. However, seepage is an active gas and fluid release process, and the gas chimney indicates the process of gas and fluid accumulation and migration; they make up an integrated overpressure system. Therefore, the seepage is classified as an overpressure-associated structure.

Figure 1. The classification of the structures developed in shallow marine sediments. (A) Seepage from bathymetry image, Vestnesa Ridge, west Svalbard margin, Norway (modified from Goswami et al. ^[29][33]); (B) Pockmark from bathymetry image, Zhongjiannan Basin, western margin of the South China Sea, China (modified from Lu et al. [23]); (C) Gas chimney from the seismic image, Cascadia margin, off the coast of Vancouver Island, Canada (modified from Wood et al. ^[30][34]); (D) Mud diapir from the seismic image, Lower Fangliao Basin, offshore SW Taiwan (modified from Hsu et al. ^[31][35]); (E) Salt diapir from the seismic image, upper Atlantic margin, SE Brazil (modified from de Mahiques et al. ^[32][36]); (F) Landslide from bathymetry image, Pearl River Mouth Basin, the northern slope of South China Sea (modified from Chen et al. [3]); (G) Polygonal fault from the seismic image, offshore Uruguay (modified from Turrini et al. ^[33][37]); (H) Tectonic fault from the sea-bottom picture, Tekirdag Basin, Sea of Marmara (modified from Armijo et al. ^[34][38]).

Diapir structures are soft-sediment deformations driven by differential gravity and compaction ^{[35][36][37][38]}[29,30,31,32]. The soft plastic layers such as mud layers and salt layers easily extrude into overlying layers because of differential forces. Therefore, the mud diapir (Figure 1D) and the salt diapir (Figure 1E) are classified as diapir structures. The mud volcano and mud mound are also included in shallow marine structures in this study. The overpressure of the gas- and fluid-rich mud sediment is a critical driving force for their formation. In this study, we classify mud volcanoes and mud mounds as mud diapirs. The two structures are regarded as the later stage of the mud diapir. When the mud diapir reaches the seafloor, a conic-like structure formed is termed a mud volcano, and a dome-like structure is termed a mud mound.

Landslides (Figure 1Figure 1F), polygonal faults (Figure 1G), and tectonic faults (Figure 1H) are driven by gravity, compaction forces and tectonic forces, respectively. All these structures have one or multiple prominent rupture surfaces; therefore, they are classified as sediment ruptures. The structural characteristics are usually different because they are not driven by a single force. The rupture surface is a reasonable connection that strings together the landslide, the polygonal fault, and the tectonic fault. In addition, the “shallow marine structure” is not restricted to a depth of less than 1000 mbsf due to the following considerations. First, the shallow marine structures are generally distributed in shallow unconsolidated marine sediments. Their structural characteristics are various and are easily influenced by many unpredictable factors. Second, some structures are developed in shallow and deep environments, and some structures are developed in a deep environment but extend into a shallow environment.