Introduction

The Mississippi Sound is an estuarine environment separated from the northern Gulf of America (formerly the Gulf of Mexico) by a chain of barrier islands. The waterbody is bordered to the north by the States of Mississippi and Alabama, and merges to the east and west with Mobile Bay and Lake Borgne, respectively (fig. 1). The sound spans approximately 100 kilometers (km) from east to west and encompasses an area of approximately 1,400 square kilometers. The five barrier islands that enclose the Mississippi Sound on the seaward side are Dauphin, Petit Bois, Horn, Ship, and Cat Islands. Except for Dauphin Island, the barrier islands are part of the Gulf Islands National Seashore. Large inlets between the islands provide connectivity between the gulf and sound.

Other than tidal and shipping channels, the bathymetry within the Mississippi Sound is shallow, with water depths typically less than 5 meters (m) (fig. 2). The water is generally brackish (Priddy and others, 1955), with freshwater supplied to the sound from major waterways such as the Pascagoula and Pearl Rivers (fig. 1). The critical blend of freshwater and saltwater within the sound creates a diverse and ecologically rich environment that supports a wide variety of marine life. Important aquaculture operations include shrimp and oyster harvesting, and the sound supports marine industries such as commercial/recreational fishing and maritime shipping.

The geology of the Mississippi Sound region is influenced by a complex interplay of sea-level fluctuations, fluvial and deltaic processes, and barrier island evolution. The present-day morphology of the sound reflects a geologic history that extends back to the Pleistocene, when sea-level fluctuations drove fluvial processes that both eroded and deposited sediments across the region. The modern barrier islands (fig. 1) developed toward the end of the last sea-level transgression (Otvos, 2001). These islands provide critical habitat for the diverse flora and fauna of the area.

The stratigraphy offshore of Hancock County, Mississippi (fig. 1), reflects the dynamic evolution of the sound and barrier island systems. Using high-resolution single channel seismic profiles (HRSPs) and sediment vibracore data, the U.S. Geological Survey (USGS) in cooperation with the U.S. Army Corps of Engineers (USACE), identified numerous features below the modern seafloor that are related to fluvial and tidal processes occurring during the last 5,000 years. For this study, seismic descriptions and core interpretations were analyzed for these features, where available. Deposits that provide the potential for sediment resources (for example, sand for coastal nourishment) were mapped for spatial extent and volume. Lastly, a coring strategy is provided to further investigate these features to determine their sedimentary characteristics.

The Mississippi Sound faces environmental challenges such as water-quality issues, habitat degradation, erosion, and the impact of sea-level rise and climate change (La Peyre and others, 2014; Morgan and Rakocinski, 2022). Efforts to protect and restore the sound's ecosystem are crucial to preserving its ecological and economic value for the region. Understanding the geology is crucial for informing coastal management decisions, hazard assessment and mitigation, and identifying potential mineral resources. The literature review completed for this study aims to provide a comprehensive overview of the geology of the Mississippi Sound by synthesizing existing research and data to identify morphologic features and processes.

Purpose and Scope

This report documents the shallow geologic framework of the Mississippi Sound and the potential for sediment resources. The literature review herein provides an outline of the Pleistocene and Holocene geology, the evolution of the Mississippi barrier islands, and the marine habitats and morphologic features within the sound. Existing reports and scientific journal articles pertaining to the geology, morphology, geography, physical processes (sediment transport, hydrology, oceanography), management, physical assessment (sediment type, biology), and habitat associated with the Mississippi Sound are provided in appendix 1. Geologic data, such as seismic data, sediment cores, and structure and elevation maps, are also identified and synthesized.

Methodology

To compile existing literature specific to the study area, search resources from the USGS library catalog were used, and more than 80 articles were identified. For each article, a list of keywords that summarize content was created and is provided in text format in appendix 1.

All HRSP data holdings acquired within the Mississippi Sound and archived by the USGS in St. Petersburg, Florida, are shown in figure 3. This dataset contains seismic data collected from 1980 to 2010 in various analog (paper prints) and digital (image) formats. The analog HRSP, formerly “boomer” data archived as paper rolls and fan-folds, has been converted to digital format, and both the former analog and existing digital data have been published in USGS series publications and data releases (Sanford and others, 2009; Forde and others, 2011a, b, 2012; Bosse and others, 2017).

A key geologic horizon within the sound, the Pleistocene-Holocene surface is relevant to regional morphologic evolution and sediment resource assessment. This unconformable surface is traceable continuously in subbottom profiles across the region and represents the last sea-level lowstand. It has been identified and mapped in four previous studies (Hollis, 2018; Adcock, 2019; Gal and others, 2021; Peoples, 2022). For this study, a regional Pleistocene surface was integrated from these efforts and is discussed in the following section.

Offshore of Hancock County in the northwestern area of the Mississippi Sound (fig. 4), HRSP data were used to identify the acoustic stratigraphy. Approximately 225-line km of HRSP data were scanned from paper copies archived at the USGS in St. Petersburg, Fla., converted to industry standard Society of Exploration Geophysicists Y Format (SEG-Y) (Barry and others, 1975), and published as a USGS data release (Bosse and others, 2018). For this study, the data were entered into seismic-data interpretation software (OpendTect version 7.0, https://dgbes.com), along with the National Oceanic and Atmospheric Administration’s National Geophysical Data Center (NGDC) bathymetric digital elevation model (NGDC, 2010), to develop a three-dimensional visualization cube (fig. 5). The HRSP lines were resampled to the bathymetric grid to align the lines to a common vertical datum, specifically the North American Vertical Datum of 1988 (NAVD 88). To ground truth the geophysical data, 81 sediment vibracore and 12 penetrometer logs from within the survey area were recovered from USACE archives and analyzed for use in this study. These logs contain descriptions of stratigraphic units within the upper 9 m of sediment (fig. 6). The depths to the tops of these units were digitized and entered into the visualization cube using the spatial information included in the logs. This process allows for direct correlation of acoustic horizons captured in the seismic data to stratigraphic information derived from the vibracore data.

To properly correlate stratigraphic depths from the core logs to the two-way travel time (TWTT) of acoustic horizons in the HRSP, a velocity analysis was performed on the data where the HRSP overlapped with the core data. It was assumed that a contrast in lithology produces a sharp reflector (significant impedance in the acoustic data displays as an amplitude peak) across the density gradient, in this case the transition from clay to sand, and that the corresponding depth in TWTT and depth to the stratigraphic horizon, in meters, can be used to calculate the sound velocity of the acoustic signal within the sediments. This velocity was used to correlate the HRSPs to the sediment cores. A strong acoustic horizon in the HRSP was identified that correlated to a specific stratigraphic horizon in the core logs (in this case, the transition from a clay unit to a sand unit occurring 3–6 m below the seafloor [mbsf]). The comparison of depth to the acoustic reflector and depth to the stratigraphic horizon between the HRSP and sediment cores is shown in table 1.

The TWTT to the reflector for 20 instances was graphed versus the depth of the stratigraphic horizon relative to sound source, which is positioned at the water surface. The cross-plot (fig. 7) produces a best-fit second-order polynomial curve that was used to calculate depth in TWTT, in milliseconds, for depth intervals in 1-m increments over the range of target depths (3–6 mbsf). Using the curve fit to the data rather than direct comparisons reduces water-column sound velocity artifacts and temporal level variations from the downcore depth correlation. The resulting velocities for each depth interval range from 1,161 to 2,471 meters per second (m/s), which are comparable to the expected range (1,491–1,836 m/s) for marine sediments (Hamilton, 1971). The average value of the range (1,878 m/s) was assumed to be the best-fit sound velocity for the sediment column. This velocity was then applied to the stratigraphic depth of the markers that have been entered into the visualization software for correlation with the HRSP.

Once the HRSP and core data were properly aligned in the visualization software, the data were interpreted to characterize the geologic framework within the study area. Where the sediment core data overlapped the HRSP, sedimentary texture was inferred to continue along the profile to the extent of the acoustic horizon. As subsurface sediment deposits were identified, their spatial distribution was mapped and extracted for volumetric analysis. These features are discussed in the analysis section of this study.

Geology of the Mississippi Sound

Pleistocene Deposits

The oldest shallow (< 50 m) stratigraphy in the Mississippi Sound is a series of transgressive and regressive sequences associated with Pleistocene sea-level fluctuations in response to glacial cycles (Anderson and others, 2016; Peoples, 2022). During interglacial periods, when sea level was near its present position, sediments associated with estuarine, deltaic, and shallow marine environments formed the coastal terraces throughout the region (Heinrich, 2006), and deltaic deposits from ancestral rivers infilled areas of the present-day sound. As sea level dropped during glacial periods, these deposits were incised by fluvial systems that meandered across exposed areas of the inner shelf, forming river valleys (Kindinger and others, 1994). As subsequent sea-level rise flooded the region, these deposits were eroded and reworked into expansive sand sheets. The river embayments filled with sediment as bayhead deltas retreated upstream (Bart and Anderson, 2004). When these interglacial and glacial-period deposits are viewed in a seismic profile, the incised valleys form distinct features that are characterized as dipping or chaotic reflectors within a sequence boundary reflector that represents the previous regressive surface (Flocks and others, 2015). The infill deposits are a combination of fluvial channel sands or muds associated with estuarine deposits (Hollis, 2018).

The stratigraphic surface representing the last sea-level lowstand is described in sediment cores as a stiff, oxidized medium gray to orange clay. The oxidized nature is indicative of subaerial exposure during lowstand conditions (Greene and others, 2007). This surface marks the transition from the Pleistocene to Holocene and is readily mapped in seismic profiles (fig. 8). Four studies have mapped the upper surface of the Pleistocene within the sound (Hollis, 2018; Adcock, 2019; Gal and others, 2021; Peoples, 2022). A compilation of the Pleistocene surface mapped in these studies is shown in figure 9. The surface deepens from east to west and seaward and it exhibits a dendritic appearance typical of fluvial systems. Sediments above this horizon represent marine-transgressive and backstepping fluvial depositional processes that occurred during the Holocene.

Holocene Deposits

Sea-level rise during the Holocene eroded the Pleistocene interfluve areas, creating a disconformity between the Pleistocene sequence boundary and subsequent marine deposits. As the Mississippi Sound embayment was initiated, transgressive deposits infilled the Pleistocene paleovalleys. When sea level neared the present position, 4–6 m of marine sands and muds (estuarine deposits in fig. 8) had been deposited within the area of the present-day sound (Twichell and others, 2013). The reworking of these marine deposits formed the first of the Mississippi Barrier Islands around 5,500 years ago (Otvos and Giardino, 2004). To the east, sediment transport from Dauphin Island and a feature known locally as the “Mobile Pass ebb-tide delta” provided sandy sediments for the evolving barrier island system. Recent studies indicate that the barrier islands formed along a change in gradient in a former Pleistocene lowstand sequence boundary (Hollis, 2018) and that recurring tidal inlets between the islands correlate with the positions of former Pleistocene fluvial valleys (Flocks and others, 2015; Gal and others, 2021). Although the current barrier island configuration is relatively young, remnants of earlier barrier island systems, such as Point Clear Island and Campbell Island, remain stranded within mainland estuaries along the Mississippi coast (Otvos, 2001).

To the west, progradation of the Mississippi River Delta influenced sediment transport across the sound. Delta lobes from the St. Bernard delta complex of the Mississippi River transgressed into the western sound and Lake Borgne areas between 4 and 2 kilo-annum (ka) (Frazier, 1967; Otvos and Giardino, 2004). These lobes introduced fine-grained sediments (for example, clays) into the system, which had formerly been dominated by active barrier island development (for example, sands). The change from an open embayment to delta marsh environment reduced wave energy and promoted marsh expansion around the barrier islands that had been developing along the mainland shoreline. Mainland adjacent islands, such as Point Clear Island, were stranded within the newly formed estuarine environment. Offshore, the T-shape of Cat Island (fig. 1) reflects this diversion in wave energy and reduced sediment supply that restructured sediment transport directions at the island (Otvos and Giardino, 2004).

The present-day Mississippi Sound is defined by the barrier island system that flanks the seaward side. The barrier islands range in length from approximately 2 to 20 km and are less than 2 km wide (fig. 2). The islands are highly dynamic; storm-induced breaching and island reconfiguration are commonly observed (Morton, 2008). Some of the breaches have evolved into tidal inlets, and tidal flow through the inlets has developed expansive flood-tidal deltas that introduce sandy sediments to the otherwise muddy estuarine deposits of the sound. An analysis of surface sediments (Upshaw and others, 1966) shows sandier sediments distributed along the barrier islands, whereas the majority of sediments within the sound are composed of silts and clays (fig. 10). The morphology of the northern (mainland) shoreline is composed of a mixture of microtidal estuaries, tidal marsh, and pocket bays (Greene and others, 2007), with remnant barrier islands immediately offshore or drowned within the marsh environment (Otvos and Giardino, 2004).

Shallow-water seagrass beds are present on the sound side of the barrier islands and along the mainland shoreline. Areas along the northern shore with harder substrates provide attachment for oyster reefs, and freshwater flow from rivers is an important contributor to reef development. The main rivers entering the sound, the Pascagoula River to the east and Pearl River to the west (fig. 1), introduce freshwater and provide nutrients for oyster development. Along with the seagrass beds and oyster reefs, sandy shorelines, such as Hancock County beach in Mississippi, provide mainland protection from storms and sea-level rise. This shoreline is experiencing erosion at a rate of about 7,600 cubic meters (m³) per year and requires regular renourishments of sand from borrow pits immediately offshore (Schmid, 2002). Sand is a finite resource, and identifying areas of potential sandy deposits for future coastal restoration is a continuing challenge. The following section of this report characterizes geologic features offshore of Hancock County in the context of potential sand-rich sediment resources.

Potential Sediment Resources of Hancock County, Mississippi

Analysis

Offshore of Hancock County, the acoustic stratigraphy was interpreted from approximately 125-line km of HRSP (fig. 4) to identify sediment deposits. The HRSP within the study area represents the top 100 m of stratigraphy, and distinct sedimentologic features are interpreted and mapped by following the subsurface horizons that persist throughout the seismic profiles. Within the Mississippi Sound region, these features are associated with buried fluvial channels, marine-transgressive deposits, and tidal deposits (Twichell and others, 2011; Miselis and others, 2014; Flocks and others, 2015). To ground truth the sedimentologic composition represented by the reflectors, direct sampling of the stratigraphy using borings and sediment cores is necessary. Within the study area, 93 vibracores and penetrometer logs were identified from the digital archives of the USACE (fig. 4). The cores were collected adjacent to the shoreline, and their distribution does not extend offshore where the majority of the HSRPs were collected. Where core data interpretations are correlated with the acoustic horizons in seismic profiles, the stratigraphic horizon can be spatially expanded using the latter. However, where acoustic facies did not have stratigraphic correlation, only assumptions about the nature of the stratigraphy could be made. In this study, four distinct sedimentologic deposits and one horizon (fig. 11) were mapped and are discussed in the following section.

Pleistocene Seismic Horizon

Throughout the study area, an acoustic reflector that shallows landward (south to north, fig. 12) is identified and mapped in all of the HRSPs. Depths to this flooding surface (ravinement), in meters below the sound-source which resides at sea level, range from 2.25 m nearshore to over 7 m offshore (fig. 13). No cores penetrate this horizon, but the depth of the reflector correlates with the Pleistocene stratigraphy observed throughout the Mississippi Sound (figs. 8–9). The strength and persistence of the return signal suggests the horizon represents a ravinement surface within the Pleistocene, possibly the top of the Prairie Formation (Otvos, 2001; Heinrich, 2006). The stratigraphy above this horizon would correspond to sediments deposited at the end of the Pleistocene and throughout the Holocene.

Holocene Lagoon Deposits

The predominant deposits within the study area are estuarine muds that accumulated within the Mississippi Sound throughout the Holocene. As the Mississippi River prograded into the area from the west to form the St. Bernard delta complex, it not only deposited thick sequences of prodelta muds (Twichell and others, 2011) but also formed a morphological transition from an open embayment to lagoonal environment. The decrease in current energy was coupled with a decrease in sand supply from the barrier island chain to the east (Otvos, 2001). Silts and clays from the fluvial systems to the north and marine muds transported by tidal currents from the Gulf of America became the dominant deposits within the sound. Sediments collected from this environment are described in the USACE vibracore logs as dark gray silty clay (fig. 6). In the seismic profiles, the lagoonal deposit is generally characterized by weak horizontal reflectors infilling a strong basal reflector (fig. 14). Depth to this basal horizon is variable but generally deepens seaward and to the southwest (fig. 15).

Fluvial Channel and Tidal Deposits (A and B)

Two features in the offshore portion of the study area (deposits A and B in fig. 11) have acoustic reflectors that resemble deposits found throughout the northern Gulf of America and were formed through fluvial or tidal processes. Parallel acoustic reflectors that are either horizontal or dipping represent fluvial valley fill, tidal deltas, or tidal inlet fill (Twichell and others, 2011; Miselis and others, 2014; Flocks and others, 2015). Without direct sampling through sediment cores, it is often not possible to determine the exact provenance of these features, but all are formed through current activity, either tidal or fluvial driven. These high-energy current processes typically move and deposit sand, although fluvial channel fill can commonly contain muds as well.

The first feature (deposit A in fig. 11) has the acoustic characteristics of a buried fluvial channel, with down-dipping reflectors forming a channel base, overlain by “infilling” horizontal reflectors (fig. 16). The chaotic pattern at the thalweg of the channel represents biogenic gas accumulation, which attenuates the acoustic return and obscures any underlying stratigraphy, a common occurrence in buried fluvial channels. During the last sea-level lowstand, fluvial channels entrenched the inner shelf (Kindinger and others, 1994; Hollis, 2018; Gal and others, 2021). As sea level rose, the channels were infilled with bayhead delta and marine deposits. No sediment cores penetrate this deposit, and direct sampling is necessary to determine the texture of the infill. The feature generally trends northeast-southwest across the study area and is mapped across consecutive HRSP lines. An isopach map of the deposit (fig. 17) shows the concave nature, with up to 9 m of channel fill within the center and thinning toward the flanks.

The second feature (deposit B in fig. 11) is characterized by acoustic reflectors down-lapping in an offshore, southerly direction (fig. 18). The deposit extends beyond the HRSP coverage; consequently, the total spatial extent of this unit is unknown. The acoustic pattern of this feature is consistent with tidal deposits seen elsewhere across the Gulf of America shelf (Twichell and others, 2011). Although no vibracores were collected within this deposit, four penetrometer logs were acquired along the western edge (fig. 11). The penetrometer probe met refusal at 4 mbsf, where it encountered a fine white sand; this depth below the seafloor likely corresponds to the top of the tidal deposit. An isopach map of deposit B (fig. 19) indicates that, within the study area, the deposit thickens up to 5 m offshore with a volume of 8.5×10⁶ m³.

Shoal Deposit

At the seaward edge of the study area, there is a deposit that resembles, in shape, a remnant barrier or shoal system (fig. 11). The acoustic signature is consistent with shoal deposits identified in the northern Gulf of America (Twichell and others, 2011; Kindinger and others, 2014), with distinct bottom and top reflectors, and chaotic or massive interior reflectors (fig. 20). No sediment cores were acquired from the deposit, so the stratigraphy can only be assumed from the chaotic nature of the seismic signal observed within the HRSP. A similar deposit was identified between the study area and Cat Island to the southeast. The deposit, described as light olive-brown sand with shell fragments (North Upper Sand deposit in Kindinger and others, 2014), contains over 92 percent fine- to medium-grained sand. Both features align with the strandplain ridges along the Hancock County shoreline and Cat Island offshore; the alignment suggests they could be submerged island remnants postulated in Otvos and Giardino (2004). The thickness of the deposit exceeds 5 m at the crest (fig. 19), with a study-area volume of 12.3×10⁶ m³.

Proposed Reconnaissance Coring Strategy to Ground Truth the HRSP

The three deposits identified in this study that have potential as sediment resources for shoreline restoration include the possible fluvial channel (figs. 16–17), tidal (figs. 18–19), and shoal deposits (figs. 19–20). The spatial and volumetric extents of these features determined in this study are shown in table 2. To ground truth the seismic reflection interpretations and assign sediment types to the seismic stratigraphy, direct sampling of the sediments is necessary. To identify shallow geologic and seafloor morphologic features in marine environments, 9-m-long sediment cores are commonly collected using a vibratory system (vibracore). Analysis of vibracores can include visual interpretation, sediment grain size measurements, and other analytic procedures that provide data and insight into the sedimentology of the samples collected and the stratigraphy. Seismic reflectors represent changes in density between lithologic units (for example, a transition from sand to silt) and are correlated with stratigraphic horizons. Once the seismic reflectors have been correlated to the sedimentology and stratigraphy in a vibracore or vibracores, the stratigraphic units can then be extrapolated across the study area using the HRSP.

For features within the study area to be considered a viable sediment resource, the deposit must be at or near the seafloor. For this study, a minimal overburden (less than 3 m) of nonsuitable sediments (for example, silts and clays) was used to optimize sediment recovery. In the Mississippi Sound, the overburden is considered to be lagoonal or estuarine muds. The deposits that best fit these criteria are the tidal deposit (B) and the shoal deposit (fig. 11). The possible fluvial deposit (A) may contain suitable sand; however, it may have significant overburden, making it a less viable source (table 2). The proposed coring strategy is broad and at a reconnaissance level, with cores centered on the HRSP to provide ground truthing for the seismic reflectors. The intent of the exploratory coring is not to provide the high-resolution stratigraphic characterization necessary for engineering and designing borrow sites, but rather to investigate large-scale features that can be delineated using HRSPs. Table 3 lists the proposed core sites with relevant information about each site, the target deposit, and the HRSP used to select the location. Thickness values for the sediment deposit were extracted from the isopach maps (figs. 17 and 19) at each potential vibracore site. Figure 21 shows the potential core site selections in map view based on the resource criteria, as discussed, targeting the fluvial/tidal features (A and B), and the shoal deposit.

Conclusion

The Mississippi Sound is a microtidal estuary located between the central gulf coast mainland and the Gulf of America. The sound is separated from the open ocean by a barrier island system with large inlets that allow tidal flow and sediment exchange between the gulf and sound. Rivers such as the Pearl and Pascagoula provide freshwater from the mainland to maintain a brackish water environment in the sound.

The physical environment of the Mississippi Sound has been shaped by fluvial and marine-transgressive processes that have occurred across the northern gulf since the Pleistocene. Former fluvial channels infilled with sediment during the last sea-level rise, while interfluve areas were eroded and incised by rivers. Dynamic sediment transport processes formed the present barrier island system, while delta lobe progradation from the Mississippi River Delta to the west restricted island development and enhanced marsh and estuarine evolution.

The sediments dispersed across the seafloor of the Mississippi Sound reflect the Holocene history, with muds accumulating across most of the sound and sandy deposits accumulating along the barrier island platform and tidal deltas. Barrier island systems and shoals that developed early in the Holocene are buried within the sound, and sand associated with the dynamic sediment processes indicative of their evolution (for example, fluvial and tidal flow) are retained in the shallow stratigraphy below the seafloor. Data from geophysical surveys and sediment cores were used to identify three deposits and evaluate their spatial extent, volume, and depth below seafloor. Lastly, the study outlined a sediment-coring strategy to further define these features.