The Cross-Hosgri slope is a bathymetric lineament that crosses the main strand of the Hosgri fault offshore Point Estero, central California. Recently collected chirp seismic reflection profiles and sediment cores provide the basis for a reassessment of Cross-Hosgri slope origin and the lateral slip rate of the Hosgri fault based on offset of the lower slope break of the Cross-Hosgri slope. The Cross-Hosgri slope is comprised of two distinct stratigraphic units. The lower unit (unit 1) overlies the post–Last Glacial Maximum transgressive erosion surface and is interpreted as a Younger Dryas (ca. 12.85–11.65 ka) shoreface deposit based on radiocarbon and optically stimulated luminescence (OSL) ages, Bayesian age modeling, seismic facies, sediment texture, sediment infauna, and heavy mineral component. The shoreface was abandoned and partly eroded during rapid sea-level rise from ca. 11.5 to 7 ka. Unit 2 consists of fine sand and silt deposited in a midshelf environment when the rate of sea-level rise slowed between ca. 7 ka and the present. Although unit 2 provides a thin, relatively uniform cover over the lower slope break of the older shoreface, this feature still represents a valuable piercing point, providing a Hosgri fault slip rate of 2.6 ± 0.8 mm/yr. Full-waveform processing of chirp data resulted in significantly higher resolution in coarser-grained strata, which are typically difficult to interpret with more traditional envelope processing. Our novel combination of offshore radiocarbon and OSL dating is the first application to offshore paleoseismic studies, and our results indicate the utility of this approach for future marine neotectonic investigations.
Assessment of earthquake hazards requires comprehensive characterization of active faults, including documentation of fault location, length, connectivity, slip rate, and rupture history. Acquiring such documentation for faults in the near offshore can be especially important given the possible impact on dense coastal populations and infrastructure. The target of this study was the slip rate of one such structure, the Hosgri fault of central California (Fig. 1). We focused on a distinct, linear, southwest-facing bathymetric slope referred to as the Cross-Hosgri slope (Fig. 2; Johnson et al., 2014). The Cross-Hosgri slope crosses and is offset by the main strand of the Hosgri fault ~30–35 km north of Point Buchon and the Pacific Gas and Electric (PG&E)–operated Diablo Canyon Nuclear Power Plant (Fig. 1).
Using global sea-level curves (Stanford et al., 2011), Johnson et al. (2014) interpreted the Cross-Hosgri slope as the relict shoreface of a now-eroded latest Pleistocene sand spit that formed during sea-level rise following the Last Glacial Maximum (LGM). The current depth of the Cross-Hosgri slope shoreface approximately coincides with eustatic sea level during a period of relatively slower eustatic rise, the Younger Dryas stadial (ca. 12.85–11.65 ka; Abdul et al., 2016), and the Cross-Hosgri slope was inferred to have been rapidly submerged and preserved during the following period of rapid eustatic rise, from about 11.5 to 7.0 ka B.P. Using high-resolution bathymetry (Fig. 2, inset), Johnson et al. (2014) plotted and analyzed closely spaced (12.5 m) normal-to-slope profiles bracketing the trace of the Hosgri fault where it crosses the lower slope break of the Cross-Hosgri slope. They documented 30.3 ± 9.4 m (mean ± 2 standard deviations) of dextral fault offset and ~70–110 cm of vertical offset. They then used the estimated Younger Dryas age of the Cross-Hosgri slope and the amount of dextral offset to infer a slip rate of 2.6 ± 0.9 mm/yr on the main strand of the Hosgri fault.
Both the depositional model of the Cross-Hosgri slope and the Hosgri slip rate in Johnson et al. (2014) were derived by correlation with a global sea-level curve. Highest-resolution chirp imagery and ground-truthing constraints from Cross-Hosgri slope sediments, including physical properties, geochronology, and petrology, were lacking. Given the limitations and uncertainty associated with the Johnson et al. (2014) approach, U.S. Geological Survey (USGS), PG&E, and the University of California at Santa Barbara collaborated through Cooperative Research and Development Agreements on a 2019 geophysical and coring program focused primarily on the Cross-Hosgri slope. The goals of the program were to better understand Cross-Hosgri slope stratigraphy and sedimentology and to obtain samples for geochronological studies. Initial results of this recent work were presented by Medri et al. (2022), who showed that the Cross-Hosgri slope is underlain by three distinct post-LGM stratigraphic units deposited in environments ranging from shoreface to midshelf and ranging in age from ca. 12.4 ka to modern, based on radiocarbon dating. Medri et al. (2022) focused on the sedimentology of a middle unit of midshelf sediments deposited between ca. 7.0 and 1.0 ka, which was interpreted as a subaqueous clinoform built by wave-supported gravity flows.
Our study primarily focused on the lowest post-LGM stratigraphic unit, which was used to estimate fault slip rate. Because the lower slope break and upper contact of this unit are the most recent observable features offset by the Hosgri fault, the timing of last deposition of this post-LGM unit is a key factor with which to constrain the most recent observable fault slip activity. We supplemented the radiocarbon dating of Medri et al. (2022) with new optically stimulated luminescence (OSL) dating of quartz sediment grains to provide further constraints on the geochronology of the Cross-Hosgri slope. We used both radiocarbon and OSL ages to generate OxCal Bayesian age models to determine the age of the shoreface deposit and the timing of burial. In addition, we used advanced processing techniques on full-waveform chirp data to greatly improve the vertical resolution and clarity of the seismic stratigraphy and fault offsets. This study documented the Cross-Hosgri slope and Hosgri fault in far greater detail and revealed that the Cross-Hosgri slope is more complex than previously envisioned. However, the additional chronology supports the fault slip rate of 2.6 ± 0.9 mm/yr proposed in Johnson et al. (2014).
HOSGRI FAULT AND CROSS-HOSGRI SLOPE
The Hosgri fault is the westernmost active fault within the broader San Andreas fault system and is the southern portion of the San Gregorio–Hosgri fault system (SGHF in Fig. 1). The San Gregorio–Hosgri fault extends along the central California coast for ~360 km and is one of the more active structures within the distributed plate boundary along the west coast of North America (Dickinson et al., 2005). Johnson et al. (2018) recognized three distinct sections of the fault based on different fault azimuths and geomorphology: (1) an ~110-km-long southern section, including the Cross-Hosgri slope, which extends from offshore Point Arguello to San Simeon, has a mean transtensional azimuth of 336° ± 8°, and is characterized by moderate coastal relief, large coastal embayments, the mouths of the Santa Maria and Santa Ynez Rivers, and a wider continental shelf; (2) an ~80-km-long middle section (the “Big Sur Bend”), which extends from San Simeon to Point Sur, has a mean transpressional azimuth of 321° ± 9°, and is marked by steep coastal relief, small coastal watersheds, and a narrow and deeply incised continental shelf; and (3) an ~170-km-long northern section, which extends from Point Sur to Bolinas, where the fault converges with the San Andreas fault. The northern section has a mean transtensional azimuth of 337° ± 6° and is generally characterized by lower coastal relief (south of San Andreas fault influence), the large coastal embayment of Monterey Bay, the mouths of the Salinas and Pajaro Rivers, and a wider continental shelf.
Based on offset piercing points, the San Gregorio–Hosgri fault slip rate over the last 10 m.y. averages ~14 mm/yr (e.g., Clark, 1997; Dickinson et al., 2005), but this rate must have decreased significantly in the late Neogene. San Gregorio–Hosgri fault lateral slip rate estimates based on late Quaternary features vary and increase from south to north, with a slip rate of ~2.6 mm/yr at the Cross-Hosgri slope location (Johnson et al., 2014). To the north at San Simeon (Fig. 1), Hall et al. (1994) and Hanson and Lettis (1994) estimated “best constrained” rates of 0.9–3.4 mm/yr and 1–3 mm/yr, respectively. Johnson et al. (2018) derived a slip rate estimate of ~3.35 mm/yr based on offset canyon heads offshore Lopez Point in the Big Sur Bend. PG&E (2014) estimated slip rates of 1.6 mm/yr and 1.8 mm/yr in Estero Bay and offshore Point Sal, respectively. These rates are notably lower than the ~11 mm/yr late Quaternary slip rate estimated by Weber (1990) for the San Gregorio–Hosgri fault at Point Año Nuevo. Increases in fault slip rate from south to north have been attributed to “adding slip” from northwest-converging faults, including the Lions Head, Casmalia, Shoreline, Oceano, Los Osos, Oceanic, Nacimiento, and Rinconada faults (Johnson and Watt, 2012; Langenheim et al., 2013; Watt et al., 2015; Colgan and Stanley, 2016; Johnson et al., 2015, 2018, 2019; Nishenko et al., 2018).
The Cross-Hosgri slope occurs at ~68–76 m water depth in the northern part of the southern San Gregorio–Hosgri fault section, ~20 km south of San Simeon and 5 km northwest of Estero Point (Figs. 1 and 2; Johnson et al., 2014). In this region, the Hosgri fault splits into an eastern main strand, which crosses the Cross-Hosgri slope (Fig. 2), and a secondary western strand, which bends to the northwest and dies out in a fold belt offshore of Piedras Blancas (Fig. 1; Johnson and Watt, 2012). The Cross-Hosgri slope has a linear, southwest-facing trend (Fig. 2), a height of 7–9 m, a length of 1700 m, and a width of 250–280 m. The feature is characterized by a slope dipping 1.6°–2.0° to the southwest at a steeper angle than the surrounding seafloor to the northeast and southwest, which dips more gently at 0.4°–0.6°. Seismic reflection chirp and coring data (see below) indicate that the Cross-Hosgri slope is underlain by post-LGM sediment and is crossed by the eastern main strand of the Hosgri fault.
High-Resolution Chirp Data
Approximately 450 km of high-resolution subbottom profiles were collected along the central California shelf onboard the M/V Bold Horizon in 2019, including seven profiles across the Cross-Hosgri slope (Fig. 2; Snyder et al., 2022). Subbottom profiles were collected using an EdgeTech 2300 SB-516 chirp system. Initially, deterministic deconvolution processing (match filtering) was conducted onboard the chirp to remove the sweep signature from the data. EdgeTech-outputted JSF formatted files were then converted to Society of Exploration Geophysicists (SEG-Y) format to access the raw correlated signal. Processing steps in Shearwater Reveal software (stable release 27 April 2022) were applied to the real (non-Hilbert-transformed) portion of the data (Henkart, 2006) and included towed depth correction (pressure sensor correction), trace balancing, multistep static (swell) removal, predictive deconvolution, automatic gain adjustment, and water column mute. Due to significant sea swell during data collection, a custom static correction workflow was designed to correctly align reflections in the full-waveform data set. The multistep static (swell) removal consisted of: (1) automated seafloor threshold picking on the envelope data, (2) median calculation and filtering of the envelope seafloor picks to produce a smoothed seafloor, (3) a three-pass cross-correlation static shift windowed around the seafloor while data were flatted to the smoothed seafloor pick, (4) transfer of the envelope static corrections to the real (full-waveform) version of the chirp data, and (5) application of a final cross-correlation static correction to the real (polarity-preserving) data. Predictive deconvolution parameters consisted of a 0.7 ms gap, 1 ms operator length, and 2.0% white noise. The deconvolution filter was designed within a 5 ms window below the smoothed seafloor picks. As the chirp was towed off the starboard side of the ship (vs. the stern), predictive deconvolution was applied to remove a short-path multiple likely caused by a reflection off the side of the hull. Horizon mapping and interpretation were done using IHS Kingdom Suite 2020 seismic interpretation software. Please see Supplemental Material File S1 for images of chirp profiles with no interpretations.1
Bathymetric data used in this study included high-resolution sonar data collected by the California State University of Monterey Bay Seafloor Mapping Laboratory (CSUMB) between 2009 and 2010 as part of the California Seafloor Mapping Program (CSUMB, 2012; Johnson et al., 2017). These data consist of a combination of sonar systems that collected data out to 5.6 km offshore (the 3 nautical mile limit of California’s State Waters). Additional bathymetric data were collected by the USGS in 2012 using a Reson 7111 multibeam sonar that extended coverage beyond the 3 nautical mile limit and included a patch along the Cross-Hosgri slope (Fig. 2; Hartwell et al., 2013).
Sediment Coring and Imaging
In total, 23 sediment cores were collected across the shelf on the 2019 research cruise using a Rossfelder P-5 vibracorer, including six cores on the Cross-Hosgri slope (HF-1 to HF-6; Figs. 2, 3, and 4), one core at ~80 m water depth located below the lower slope break of the Cross-Hosgri slope (HF-7), and one core located to the south at 113 m near the shelf break (HF-12; Fig. 2; Snyder et al., 2022). Cores longer than 1.5 m were split into multiple sections to permit scanning in a Geotek rotating X-ray computed tomography (RXCT) system. Down-core axial slices from the computed tomography (CT) scans were used to derive coronal (x-z) and sagittal (y-z) planar images for each core segment (Fig. 3; Snyder et al., 2022). Cores were split and then photographed using the Geotek multisensor core logger (MSCL) system. P-wave velocity was measured using the MSCL on unsplit cores HF-1, HF-3, HF-5, and HF-7, providing a mean velocity of 1617 m/s.
Thirty radiocarbon samples for geochronological analysis were collected from seven cores collected in the Cross-Hosgri slope region (Table 1; Snyder et al., 2022; Medri et al., 2022). These samples included 23 gastropod shells, three bivalve shells, and six wood fragments. Gastropods that showed no evidence of reworking were selectively sampled for dating, and species were evaluated to assess habitat (Medri et al., 2022). Radiocarbon ages were determined using atomic mass spectrometry at the University of California–Irvine W.M. Keck carbon cycle accelerator mass spectrometer (KCCAMS) facility. The 14C ages calculated from shells used the Marine 20 calibration curve of Heaton et al. (2020) for calibration, whereas wood fragments used the IntCal atmospheric calibration curve of Reimer et al. (2020) within the Calib 8.2 program (Stuiver et al., 2022).
Optically Stimulated Luminescence Dating
To supplement the radiocarbon dating and provide an independent verification, we applied OSL dating to 10 sediment samples to determine when quartz grains were last exposed to daylight (Table 2; Murray et al., 2021). Following the methods of Nelson et al. (2019), we split cores and extracted samples for equivalent dose and dose rate measurements under dark-room conditions. Elemental concentrations were determined by inductively coupled plasma–mass spectrometry on samples immediately above, below, and surrounding the equivalent dose sample, and dose rates were calculated using the Dose Rate and Age Calculator tool (Durcan et al., 2015). We applied standard methods to determine the equivalent dose (i.e., Murray et al., 2021) using the single-aliquot regenerative dose protocol (Murray and Wintle, 2000) on ≤2-mm-sized masked aliquots of quartz sand. This means that for each aliquot, fewer than 300 grains were analyzed in an averaged luminescence signal (Duller, 2008). OSL characteristics of the samples were favorable for this method, with a dominant fast component (Durcan and Duller, 2011), and high percentages of aliquots passed our rejection criteria (Wintle and Murray, 2006). We did not observe substantial evidence of partial bleaching, which would be manifested by high overdispersion and skewed dose distributions (e.g., Fig. 5). We calculated ages using the central age model (CAM) and minimum age model (MAM) from functions in the “luminescence” package for the programming language R (Kreutzer et al., 2012). We found that the CAM produced the best agreement with the radiocarbon ages and thus argue that this model produces the most-accurate, best-bleached exposure age estimates for the sampled core sediment. For additional details and figures related to the methodology, please see the associated Supplemental Material File S2 and Snyder et al. (2022).
Bayesian Age Modeling
We constructed several Bayesian age models with different assumptions and commands for two composite stratigraphic sections, HF-1–HF-2 and HF-3–HF-4 (Table 3; Figs. 3 and 4), to (1) determine a representative age for unit 1 (Table 3) and (2) refine the timing of burial of unit 1 (Tables 4 and 5; contact that separates unit 1 from unit 2). To construct the composite HF-1 section, ages were combined from the two cores by stretching the thinner unit 2 in HF-1 to match the thickness of HF-2, whereas for unit 1, the ages were combined by using the measured depth below the unit 1-2 contact (Fig. 3). The same approach was used for the HF-3–HF-4 composite stratigraphic section (Fig. 4).
For every OxCal age model run (Supplemental Material File S3), we accounted for out-of-order ages by incorporating the “Outlier” command lines, which allow OxCal to adjust the influence of suspect ages into the modeled ages (output) accordingly. To derive a representative age for unit 1, we employed the OxCal Sequence modeling approach and used the “Combine” command to incorporate both OSL and radiocarbon age constraints to model a reduced average unit age (i.e., smaller error than what unmodeled ages would suggest). Additionally, although we followed the approach of Medri et al. (2022) regarding the use of a DeltaR value of 0 ± 0, we considered the impact that alternative DeltaR values might have on age results by incorporating a range of DeltaR values, 182 ± 15 and −128 ± 15, which were found on the Calib map searchable database (Calib.org/marine) for areas roughly 20 km away from our site. To refine the timing of unit 1 burial, we employed OxCal Sequence and OxCal P_Sequence, each with different assumptions, to assess the impact of estimated ages that may be imposed based on model choice. OxCal Sequence only uses the position of age constraints within the stratigraphy to build timing relationships, whereas OxCal P_Sequence models a sedimentation rate based on sample depths. We expected that the sedimentation rate–based model, OxCal P_Sequence, would provide more precise age estimates than the position-dependent model, OxCal Sequence. However, due to the age constraint reversals (with respect to assigned depths) within unit 1, the OxCal P_Sequence failed to run to completion. To circumvent the P_Sequence failure, we reassigned the depths of ages out of order to fit in a youngest-to-oldest order, which contributed to low outlier results. See Supplemental Material File S3 for additional details on modeling approaches.
Samples from sandy intervals within cores were collected and processed to generate fractions of uniform fine grain size (125–250 μm). Steps included wet sieving and air and oven drying, followed by dry sieving. Standard thin sections with clear epoxy impregnation and potassium feldspar stain were examined petrographically and point counted, 323–408 points per sample (Table 6), to compare units 1 and 2 and provide insights on depositional environment. Additional details are provided in Snyder et al. (2022).
Here, we present results that establish the stratigraphic and chronologic development of the Cross-Hosgri slope and characterize fault activity. Within the Cross-Hosgri slope, Medri et al. (2022) defined three seismic stratigraphic units (S1, S2, S3) above a post-LGM transgressive surface of erosion based on seismic reflection data, coring, and geochronology. Their S1 unit is here labeled “unit 1,” and their S2 and S3 units are here grouped into our “unit 2,” as described below. The measured mean P-wave velocity of 1617 m/s was used for time-to-depth conversion for thickness estimates from the chirp imaging.
Transgressive Surface of Erosion
Global sea level was ~120–130 m lower than present ca. 21 ka during the LGM (Stanford et al., 2011), at which time most of the central California shelf was emergent, and the shoreline was at or near the current shelf break (Johnson et al., 2019). Landward migration of the shoreline across the ~6-km-wide emergent shelf in this area is marked by a prominent, shelf-wide, unconformity (Johnson and Watt, 2012; Johnson et al., 2019, 2020). This unconformity, a transgressive surface of erosion in seismic stratigraphic nomenclature (e.g., Catuneanu, 2006), is imaged on chirp seismic reflection profiles as a relatively continuous moderate- to high-amplitude reflection bounding and truncating reflection-free and weakly reflective material below and an overlying zone of moderate- to high-amplitude discontinuous reflections (Figs. 6, 7, 8, and 9). The transgressive surface of erosion can be traced across the region from the midshelf to the outer shelf on chirp profiles and was penetrated in four cores collected at water depths of 101–113 m. In core HF-12, the surface is represented as a shell lag that yielded both pre-LGM and post-LGM radiocarbon dates, indicating erosion and mixing of sediment along the transgressive surface (e.g., Fig. 9). Sediment below the shell lag deposit is composed of predominantly black sands that yielded an OSL age of 55.9 ± 2.9 ka, indicating that the base of the shell lag is an erosional surface. Such shell lags commonly occur along and above transgressive erosion surfaces as discontinuous scour fills of variable thickness (Catuneanu, 2006), and we infer that they are the source of the high-amplitude reflections common in the lower part of unit 1 (see below).
Unit 1—Shoreface Deposits
Unit 1 (unit S1 of Medri et al., 2022) overlies the transgressive surface of erosion, forming the lower stratigraphic unit within the Cross-Hosgri slope. Unit 1 is characterized by variable seismic facies including parallel to locally divergent, low- to high-amplitude reflections and homogeneous “reflection-free” zones (Figs. 6, 7, and 8). Reflections commonly dip offshore ~1.8°, similar to the slope of the Cross-Hosgri slope seafloor (e.g., HFC-5; Fig. 6). On some profiles, dips near the base of the unit appear to be more parallel to the transgressive surface of erosion below (e.g., HFC-3; Fig. 6). In addition, at least one internal unconformity was imaged within unit 1 on HFC-3 (Fig. 6). Laterally, the ~1300-m-long chirp profile section that extends across the mid–Cross-Hosgri slope (HFC-25A, HFC-25B; Fig. 8) shows that the lower boundary (base) of the unit has an erosional, channeled surface, whereas the upper boundary (top) is mostly smooth. The thickness of unit 1 ranges laterally from ~100 to 310 cm, with the variation predominantly due to the channeled relief on the lower erosional boundary. The lower and upper boundaries of unit 1 also converge in both the onshore (upslope of the Cross-Hosgri slope) and offshore (downslope of the Cross-Hosgri slope) directions, so that the stratigraphic horizon forms a distinct lens with maximum thickness located beneath the Cross-Hosgri slope (Figs. 6 and 7).
Cores from the Cross-Hosgri slope were not long enough to penetrate to the transgressive surface of erosion at the base of unit 1. However, multiple cores preserved samples of the uppermost (~50 cm) sediments of unit 1. This included the lower 12 cm of core HF-1, the lower 24 cm of HF-2 (Fig. 3), the lower 29 cm of core HF-4, the basal core cutter (bottom portion of core) of HF-3 (Figs. 4 and 6A), and the lower 50 cm of core HF-7, located ~80 m below the lower slope break of the Cross-Hosgri slope (Fig. 7B). Medri et al. (2022) described the sediment in unit 1 as the “black sand facies,” composed of well-sorted fine-grained sand (mean grain size = 130 μm) with less than 5% silt, no clay, and sparse shell fragments. They reported that shells within unit 1 included Clathurella canfieldi and Truncatella californica, which prefer a sandy habitat, among surf grass roots in the upper intertidal zone (Guz, 2007). Also found was Turitella cooperi, which can occur at intertidal to midshelf water depths.
Seismic, sedimentological, and faunal data indicate that unit 1 was deposited in a shallow-water (<20 m) shoreface environment like modern shorefaces along the high-energy California coast (e.g., Barnard et al., 2007, 2009; Medri et al., 2022). Seaward-dipping reflections (Figs. 6 and 7) indicate primary depositional slope and progradation. Sediment texture (well-sorted fine sand; Medri et al., 2022), the relative lack of infauna, and the significant proportion of heavy minerals (~8.4%; see below) in the sand are also consistent with a high-energy depositional environment.
We obtained nine radiocarbon dates and five OSL dates from unit 1 to constrain its age (Tables 1 and 2). Eleven samples were from the upper 28 cm of unit 1 beneath the Cross-Hosgri slope in cores HF-1, HF-2, HF-3, and HF-4. Three samples were from the upper 50 cm of unit 1 in core HF-7, located ~80 m below the lower slope break of the Cross-Hosgri slope. Table 1 provides the range and mean of calibrated radiocarbon dates; the overall mean age of the nine calibrated radiocarbon dates is 10.8 ka, with a variation in mean ages ranging from 9.7 to 12.0 ka The mean age of the five CAM OSL dates is 11.8 ka, with a range of 12.4 ± 0.4 ka to 10.6 ± 0.5 ka, and four of the dates fall between 12.4 ka and 11.8 ka. Bayesian age modeling of the HF-1 and HF-2 composite section (Fig. 3) using the OxCal Sequence model with a DeltaR value of zero resulted in a calibrated range of 11.9–11.6 ka, with a mean of 11.7 ka. The same approach and DeltaR used for the HF-3 and HF-4 composite section (Fig. 4) yielded a calibrated age range of 11.1–9.6 ka, with a mean of 10.8 ka (Table 3). Taking the weighted mean of the two composite mean ages resulted in a calibrated age of 11.7 ± 0.1 ka for the upper 50 cm of unit 1. Geochronological data provide a good fit for deposition near or immediately after the end of the Younger Dryas stadial (ca. 11.65 ka), a period in which the rate of eustatic sea-level rise significantly declined (Stanford et al., 2011; Abdul et al., 2016). All the dates are from the upper 50 cm of a unit that is as thick as 310 cm. Four of the five OSL dates and two of the nine radiocarbon dates fall within the Younger Dryas time interval, five of the seven remaining radiocarbon dates are slightly younger, between ca. 11.0 and 10.6 ka, and two dates are ca. 9.8–9.7 ka. The younger seven radiocarbon dates are from the uppermost part of the unit and likely reflect late–and post–Younger Dryas physical and biological reworking (e.g., burrowing, grazing) of the upper part of the unit.
Unit 2—Midshelf Deposits
Unit 2 overlies unit 1 beneath the Cross-Hosgri slope (Medri et al., 2022). The lower contact is marked by a discontinuous reflection that coincides with an upward transition from moderate- to high-amplitude reflections at the top of unit 1 to a zone of diffuse, seafloor-parallel, low- to medium-amplitude reflections that characterize unit 2 (Figs. 6, 7, and 8). The lower contact of unit 2 is parallel to low-angle discordant, appearing to locally truncate reflections in the upper part of unit 1 on some profiles (e.g., Figs. 7A and 8A). The top of unit 2 is the seafloor.
Unit 2 is up to 300 cm thick, reaching a maximum thickness on the upper Cross-Hosgri slope (Fig. 6A). Both chirp (Figs. 6 and 7) and coring data indicate that unit 2 thins markedly downslope, nearly pinching out at the lower slope break. Paired cores HF-5 and HF-6 on the upper Cross-Hosgri slope penetrated a complete 300-cm-thick section of unit 2. Paired cores HF-3 and HF-4 on the midslope and paired cores HF-1 and HF-2 on the lower slope penetrated complete 140-cm-thick and 80-cm-thick sections, respectively (Figs. 3 and 4). Core HF-7, located ~80 m below the lower slope break of the Cross-Hosgri slope, penetrated a complete 30-cm-thick section of unit 2. Chirp profiles (Figs. 6, 7, and 8) confirm this downslope thinning and show variable unit 2 thinning across the Cross-Hosgri slope.
Medri et al. (2022) subdivided unit 2 into three distinct subunits based on their sediment characteristics. The lowest subunit consists of 50% sand, 40% silt, and 10% clay, and it contains abundant shell fragments throughout. Strata consist of 10–20-cm-thick fining-upward sequences bounded by erosion surfaces. Within these sequences, sediments are parallel and ripple cross-laminated. Shell fragments are abundant, as are whole shells of gastropods (Amphissa versicolor, Callianax baetica) that commonly occur in midshelf water depths (~40–70 m). This lower subunit thins from 200 cm in HF-5 on the upper slope to 70 cm in HF-3 on the midslope to 60 cm in HF-1 on the lower slope (Figs. 3, 4, and 6A). This subunit is not present in core HF-7, located ~80 m below the lower slope break (Figs. 2 and 7B).
The middle subunit of unit 2 consists of thin (~2-cm-thick) beds of sandy shell hash composed of granule- to cobble-sized shell fragments in a sandy matrix, bounded by thin beds of parallel-laminated sand. This subunit forms one distinctive, low-angle, ~250-m-wide channel, filled with relatively higher-amplitude reflections, imaged only on the eastern part of the Cross-Hosgri slope, the base of which is highlighted with a yellow horizon on chirp profiles (Figs. 6A and 8A). Where it does occur, the thickness of this subunit decreases from 85 cm in core HF-5 on the upper Cross-Hosgri slope to 30 cm in HF-3 on the middle Cross-Hosgri slope (Fig. 4), and it is not present in core HF-1 (Fig. 3) on the lower Cross-Hosgri slope or in core HF-7, ~80 m below the lower slope break of the Cross-Hosgri slope. Impedance contrasts associated with the contrasting “hard” shell fragments and “softer” interbeds of fine-grained sediment are the likely source of the discontinuous, moderate- to high--amplitude reflections that characterize this laterally confined subunit.
The upper subunit of unit 2 consists of massive sandy silt with local burrows and scattered shell fragments. This subunit occurs in all the cores, ranging in thickness from 15 cm on the upper Cross-Hosgri slope in core HF-5 to 40 cm in core HF-3 on the middle Cross-Hosgri slope, to 20 cm in core HF-1 on the lower slope, to 30 cm in core HF-7 ~80 m below the Cross-Hosgri slope lower slope break. The contact between the middle and upper subunits does not create a consistently mappable impedance contrast on the chirp profiles, and thus it was not mapped.
Medri et al. (2022) proposed that unit 2 nucleated on the preexisting relict of a latest Pleistocene (Younger Dryas) shoreface. They inferred that sediment transport of the lowest subunit was initiated on the midshelf by winter-storm waves, with deposition occurring on the Cross-Hosgri slope as wave-supported gravity flows. The overlying shell-hash subunit was interpreted as wave-winnowed shelf deposits. The lowest unit 2 subunit thins markedly downward on the Cross-Hosgri slope, and the middle shell-hash subunit pinches out both downward and laterally on the Cross-Hosgri slope (Figs. 6, 7, and 8). Medri et al. (2022) interpreted the upper unit 2 sandy silt subunit as a young drape that was deposited by suspension settling on the Cross-Hosgri slope and adjacent shelf.
We obtained 20 radiocarbon dates and three OSL dates from unit 2 to constrain its age (Tables 1 and 2). The calibrated radiocarbon dates ranged from ca. 7.4 ka to modern, and the OSL dating yielded ages of 8.6–2.5 ka. We excluded consideration of one out-of-sequence radiocarbon date from HF-5 (mean calibrated age of 9417 yr B.P.) because seven deeper samples from this core yielded younger, in-sequence ages, from 7.4 to 3.4 ka. We also excluded one radiocarbon date from HF-1 (mean calibrated age of 4125 yr B.P.), as this age appears to be out of sequence.
In addition, we ran OxCal Sequence and OxCal P_Sequence models (Supplemental Material File S3) for the two composite sections (Figs. 3 and 4) to determine the age of the unit 1-2 contact, and these results are shown in Tables 4 and 5. Using a DeltaR value of zero, the Sequence model for the HF-1 and HF-2 composite section produced a calibrated median age of 9.1 ka, with a range of 10.6–6.9 ka. In contrast, the P_Sequence model generated a calibrated mean age of 9.6 ka, with a range of 10.7–7.9 ka. For the HF-3 and HF-4 composite section, the OxCal Sequence model (DeltaR = 0) produced a calibrated median age of 8.6 ka, with a range of 9.6–7.4 ka. The P_Sequence model for the same composite section outputted a calibrated median age of 9.7 ka, with a range of 10.9–6.8 ka. Taking the weighted mean of the two composite Sequence model mean ages resulted in a calibrated age of 8.7 ± 0.5 ka, whereas the weighted mean of the P_Sequence model was 9.6 ± 0.6 ka.
Petrographic analyses of 14 samples of fine-grained sand from units 1 and 2 are summarized in Table 3 and Figure 9. The siliciclastic framework grain proportions of unit 1 and unit 2 were nearly identical (Fig. 10; mean quartz–feldspar–total lithics [QFLt] = 30-18-52), with slightly more variation in the upper unit. The sand is derived from the varied bedrock in the Franciscan Complex in the adjacent, southern Santa Lucia Range (Graymer et al., 2014). Prominent Franciscan lithologies in this source area include mélange, sandstone, graywacke, conglomerate, greenstone, diabase, chert, serpentinite, and glaucophane schist.
Although the framework composition of the siliciclastic component of units 1 and 2 was similar, there were significant differences in the proportions of bioclasts and heavy minerals between the two units. Bioclasts consisted entirely of shell fragments in unit 1, but unit 2 included both shell fragments and common whole shells (including foraminifera tests). Mean bioclast proportion in unit 1 and unit 2 was 5.3% and 31.9%, respectively. Heavy minerals identified based on color (plain light), relief, pleochroism, and birefringence included a mix of amphibole (e.g., hornblende) and pyroxene (e.g., augite), and less common mineral types including epidote, sphene, and magnetite. The mean proportion of heavy minerals in unit 1 and unit 2 was 8.4% and 2.2%, respectively. Because of their specific gravity, heavy minerals should have increased presence in a high-energy shoreface environment (unit 1; Roy, 1999), which is also not hospitable for significant shell-bearing fauna. In contrast, the inferred midshelf environment of unit 2 should provide a less energetic location that is hospitable for shell-bearing fauna (the source of bioclasts) and is a less likely location for heavy minerals, given the need for significant offshore sediment transport.
Hosgri Fault Characterization
The chirp profiles that cross the Cross-Hosgri slope provide ultrahigh-resolution imagery of the Hosgri fault in the upper 1–6 m of the subsurface. Thus, the chirp seismic profiles are more comparable in scale to road-cut or trench exposures than to seismic reflection profiles generated by other high-resolution seismic reflection systems such as minisparkers (e.g., Johnson and Watt, 2012; Kluesner et al., 2019). The appearance of the fault varies significantly in the chirp imagery within the small Cross-Hosgri slope area (Figs. 2, 7, and 8) as it cuts through different lithologies and stratigraphy at different orientations. Although most motion on the Hosgri fault is strike slip, the transgressive surface of erosion and the contact between units 1 and 2 provide markers for documenting apparent vertical fault slip; these markers do not provide information on the more significant strike-slip offset along the fault.
The Hosgri fault cuts across the Cross-Hosgri slope on three of the four slope-normal chirp profiles (Figs. 6 and 7). Just below the lower slope break on profile HFC-3 (Figs. 2 and 5B), the Hosgri fault is imaged as a sharp contact that warps reflectors in unit 1 and vertically offsets (up to the west) the transgressive surface of erosion by ~110 cm (13.6 ms two-way traveltime [TWTT]) and the shallow (~30 cm deep) unit 1-2 contact by ~60–70 cm (7.4–8.7 ms TWTT). Based on bathymetric slope profiles, Johnson et al. (2014, their fig. 11) previously estimated ≥70 cm (as much as 130 cm) of up-to-the-west vertical offset on the lower slope break along the Hosgri fault. Profile HFC-4 imaged the Hosgri fault where it crosses the Cross-Hosgri slope ~20 m below the upper slope break (Figs. 2 and 7A). The fault is again imaged as a sharp near-vertical plane, offsetting the transgressive surface of erosion and the unit 1-2 contact by ~120 cm (14.8 ms TWTT) and 60 cm (7.4 ms TWTT), respectively. Profile HFC-2 (Figs. 2 and 6B) crosses the Hosgri fault ~60 m above the upper slope break of the Cross-Hosgri slope, imaging the fault as an ~120-m-wide zone of chaotic reflections across which reliable vertical offsets cannot be determined.
Vertical offsets of the unit 1-2 contact, along with sediment ages, indicate the Hosgri fault has been active in the last ~7000 yr. The lack of continuous, traceable internal reflections within unit 2, other than a suppressed short-path multiple, limits our ability to discern and measure vertical offsets from any single post–7 ka earthquakes. The observation that the Hosgri fault does not vertically offset the seafloor on any of the profiles likely indicates that shelf processes (deposition and erosion) have been sufficient to smooth over any vertical relief that might have been generated during the most recent earthquake(s), although bathymetric vertical relief from older (older than ca. 7.0 ka) slip events is evident along the lower slope break (Johnson et al., 2014).
The high-resolution chirp imagery taken across the Cross-Hosgri slope revealed that unit 1 exhibits seismic characteristics consistent with a prograding, high-energy shoreface deposit, such as seaward-dipping reflections, onlap/downlap terminations, and variable amplitudes (Figs. 6 and 7). A high degree of seismic variability is present within unit 1 across the Cross-Hosgri slope, and at least one internal unconformity is present on HFC-3 (Fig. 6B), both of which are common seismic characteristics of clinoform sediment packages on continental shelfs (Sheriff, 1980). Both dip profiles and perpendicular profiles across the Cross-Hosgri slope revealed reflections at the top of unit 1 that appear to be truncated in certain regions (Figs. 7 and 8), suggesting that the unit 1-2 boundary most likely represents an erosional unconformity.
The Bayesian age modeling conducted on composite sections of cores HF-1–HF-2 and HF-3–HF-4 resulted in a calibrated weighted mean age (DeltaR = 0) of 11.7 ± 0.1 ka for unit 1, which falls within the later portion of the Younger Dryas stadial (ca. 12.85–11.65 ka; Stanford et al., 2011; Abdul et al., 2016). Cores HF-1 through HF-4 only penetrated the upper ~20 cm of unit 1, whereas core HF-7 sampled >70 cm into unit 1 (Supplemental Material File S1). A sample near the bottom of the core in HF-7 yielded a calibrated mean radiocarbon age of ~12.0 ka, and the OSL age of sediment taken from the same interval yielded an age of 12.4 ± 0.4 ka (Tables 1 and 2). This pattern of older sediments with increasing depth below the unit 1-2 contact suggests that the ages of deposits within unit 1 below the depth of our cores extend to the beginning of the Younger Dryas stadial (ca. 12.85 ka).
The OxCal Sequence and P_Sequence modeling of the unit 1-2 contact, which used dates from above and below the contact (Figs. 3 and 4) and a DeltaR value of zero, produced a calibrated median age range from 9.7 to 8.6 ka, with individual calibrated ages ranging from 10.9 to 6.8 ka. As mentioned in the Results, we believe post–Younger Dryas (younger than ca. 11.65 ka) radiocarbon dates in the upper part of unit 1 likely reflect physical and biological reworking/mixing (e.g., burrowing, grazing), which is consistent with the observed sediment mixing across the unit 1-2 contact (Figs. 3 and 4). Sediments sampled at the base of unit 2, just above the unit 1-2 contact, provided a calibrated radiocarbon mean age range of 7.4–6.3 ka, indicating that deposition of unit 2 and preservation of the unit 1-2 boundary happened around 7000 yr ago.
The comprehensive geophysical, sedimentological, and geochronological data reported here and the results of Medri et al. (2022) provide the basis for a substantial revision of the Cross-Hosgri slope depositional model presented by Johnson et al. (2014). These results suggest the following Cross-Hosgri slope depositional history, with key stages shown in Figure 11.
(A) Rapid sea-level rise from the LGM (ca. 21 ka) to the beginning of the Younger Dryas stadial (ca. 12.8 ka) provided significant accommodation space for available coastal sediment supply (Fig. 11A). A transgressive surface of erosion was generated at the high-energy shoreline, preserved at least locally as a thin shell lag (Fig. 9), which is the likely source of a nearly continuous, moderate-amplitude reflection on chirp data (Fig. 9). Clastic sediment (reworked beach deposits) derived from the coastal landscape were eroded and redeposited offshore in deeper water as the shoreline migrated landward.
(B) Sea-level rise slowed considerably during the Younger Dryas stadial, which extended from ca. 12.85 to 11.65 ka (Stanford et al., 2011; Abdul et al., 2016). As a result, landward migration of the transgressive surface and shoreline slowed, and a prograding beach and shoreface developed (Fig. 11B). The lower slope break of the Cross-Hosgri slope in unit 1 occurs at depths of ~73 m below modern sea level, representing the base of the shoreface. Given the presence of an emergent rocky point at this sea level at the west end of the Cross-Hosgri slope (Hosgri Ridge on Fig. 2), the prograding beach may have formed a sand spit bounded landward by a lagoon, as suggested by Johnson et al. (2014, their fig. 7). Johnson et al. (2014, their fig. 6) cited the Bolinas area (Fig. 1) along the northern California coast (Cochrane et al., 2015) as a modern analogue, noting that the Bolinas Point promontory protects Bolinas Lagoon and east-trending Stinson Beach from maximum Pacific Ocean wave energy.
The preserved portion of this shoreface forms unit 1 of the Cross-Hosgri slope, consistent with the sedimentological, petrographic, and geochronological data outlined above. We estimate that the water depth of the slope break at the base of the shoreface unit was ~8 m, based on the depths of southwest-facing shorefaces along the California coast that are similarly bounded and somewhat protected from highest wave energy by bedrock uplifts to the northwest. These include: (1) the area on the southeast flank of Point Estero (Fig. 1, insert), where two modern shoreface profiles ~2000 m apart (E-1, E-2) mapped by Dingler et al. (1982) extend to depths of 6 and 8 m; and (2) the north end of Bodega Bay, bounded to the northwest by Bodega Head, where Johnson et al. (2019, their fig. 6) imaged two submerged Upper Holocene shorefaces 7 and 8 m high. On more exposed portions of the California coast, shorefaces commonly extend to water depths of 10–15 m (Dingler et al., 1982; Barnard et al., 2007, 2009). We therefore infer that sea level was ~65 m below present at ca. 11.7 ka when unit 1 deposition ended, representing an important data point for reconstructing post-LGM sea-level rise along the central California coast.
(C) Sea level rose rapidly between 11.65 and 7.0 ka (Stanford et al., 2011; Reynolds and Simms, 2015). This change in base level provided abundant accommodation space for deposition of coastal sediment on what is now the inner continental shelf, effectively shutting down sediment supply to the progressively deeper Cross-Hosgri slope. The upper part of the Younger Dryas–shoreface complex was eroded and reworked by wave energy in the earliest part of this time period, forming the erosional surface on seismic reflection profiles that represents the contact between units 1 and 2 (Figs. 6, 7, and 8). The sediment-starved, wave-erosion surface and underlying uppermost part of unit 1 were then colonized by shallow-water fauna, providing the material that yielded radiocarbon dates slightly younger than the Younger Dryas depositional age of unit 1 (Table 1). The wave-erosion surface converges with the underlying transgressive surface of erosion both seaward and landward of the Cross-Hosgri slope, so that the unit 1 shoreface was buried by younger sediment and preserved as an elongate lens.
(D) From ca. 7.0 ka to the present, the rate of sea-level rise dramatically decreased to ~1 m/1000 yr (Reynolds and Simms, 2015). The effects of this decrease included: (1) a substantial decrease in the rate of creation of new sediment accommodation space, and (2) an associated increase in the time available for offshore shelf-sediment transport. The net result was movement of coastal sediment to midshelf environments and deposition of unit 2 midshelf deposits as a low-angle clinoform above the relict Cross-Hosgri slope shoreface (Medri et al., 2022).
Hosgri Fault Slip Rate and Earthquake History
Determining the slip rate of a strike-slip fault requires documentation of the amount of lateral slip of a piercing point and the time period over which that slip occurred. Johnson et al. (2014) showed that the lower slope break of the Cross-Hosgri slope was a mappable, linear geomorphic feature that crossed the Hosgri fault, forming a unique piercing point (Fig. 2, inset). Understanding the development and history of the Cross-Hosgri slope lower slope break (Fig. 10) is thus essential for developing and (or) revising a Hosgri fault slip rate.
Previously, Johnson et al. (2014) inferred that the Cross-Hosgri slope was underlain by a relict Younger Dryas (latest Pleistocene) shoreface deposit (i.e., unit 1 of this study), and they noted lateral offset of both the lower and upper slope breaks. The offset was determined by plotting data and regression lines obtained from normal-to-slope profiles located 12.5 m apart (Fig. 2; Johnson et al., 2014). They restricted the slip-rate analysis to a set of 34 (of 93) points located on slope profiles from 225 m west of the fault trace to 200 m east of the fault, to limit the influence of slope curvature and (or) the depositional or erosional irregularities that occur along the lower slope break farther from the fault. They considered the lower slope break (the inferred base of the paleoshoreface) as a far more reliable piercing point because, as sea level rose in the latest Pleistocene, it would sink earlier below wave base and thereby would have experienced less reworking by storm waves and subsequent sedimentation. They measured 30.3 ± 9.4 m of lateral offset of the lower slope break based on the slope profiles, an amount similar to their estimate (34 m) based on a simpler analysis of digital elevation models and slope maps derived from high-resolution bathymetric surveys (Johnson et al., 2014).
The new data presented in this study show that the Johnson et al. (2014) analysis significantly underestimated the amount of erosion of the latest Pleistocene unit 1 shoreface deposits as Johnson et al. were not aware of the thickness and sedimentology of the overlying middle to late Holocene midshelf deposits, which had yet to be sampled (Medri et al., 2022). This new information was incorporated in the depositional model of Figure 11, providing essential context for understanding the origin and history of the lower slope break and its viability as a piercing point.
Together, the chirp (Figs. 6, 7, and 8) and core data (Figs. 3 and 4) indicate that the lower slope break represents the base of the unit 1 shoreface (Figs. 11B and 11C), covered by downslope-thinning unit 2 shelf deposits (Fig. 11D). In the 425-m-wide local area Johnson et al. (2014) used to measure fault offset, unit 2 thickness decreases downslope from ~108–124 cm (87–100 ms TWTT) thick along the HFC-25b profile (Fig. 2) to ~81–88 cm in cores HF-1 and HF-2 ~20 m farther downslope (Figs. 2 and 3). Unit 2 is ~50–60 cm thick at the lower slope break and thins to ~30 cm approximately 80 m seaward of the lower slope break on the Cross-Hosgri slope in core HF-7 (Medri et al., 2022). Core data suggest that at the lower slope break, the 50–60 cm interval of unit 2 primarily consists of the sandy silt facies and hemipelagic suspension deposits that are younger than ca. 700 yr B.P. (Medri et al., 2022, their fig. 5). Although this thin sandy silt facies drapes and probably slightly smooths the lower slope break (Fig. 11D), we do not think it compromises this distinct geomorphic feature as a piercing point.
Despite the density of chirp profile lines and the number and location of cores collected, we could not quantify the amount (if any; see Fig. 8B) of variation in unit 2 thickness across the local Cross-Hosgri slope area that was used to determine lateral fault slip (Fig. 2). For our slip rate analysis, we therefore assumed that the unit 2 thicknesses outlined above are uniform in this local, fault-adjacent area. Even if we ignored/removed the thin unit 2 cover, it would not change the locations of the lower slope break relative to one another on bathymetric slope profiles, so the amount of estimated lateral offset of this linear feature determined by Johnson et al. (2014) would remain valid. Given issues with small (~1°) slope changes, projections, and rounding errors, Johnson et al. (2014) estimated that uncertainties in locating the lower slope break based on slope profiles could be as much as 10 m for some data points, but that the effects of such errors were minimized by analyzing large numbers of slope profiles at tight profile spacing. From our assessment, it seems possible that undetected variations in unit 2 thickness, possibly modified during erosion between unit 1 and unit 2 deposition, could lead to greater uncertainty in locating the minimally buried base of the latest Pleistocene shoreface, but that increase cannot be quantified with the current data.
The OSL and radiocarbon dates reported herein strongly support a Younger Dryas age, ca. 12.8–11.7 ka, for the latest Pleistocene shoreface (unit 1 above), with the end of shoreface deposition resulting from rapid sea-level rise occurring at the onset of meltwater pulse 1B (e.g., Alley, 2000; Stanford et al., 2011; Liu and Milliman, 2004; Abdul et al., 2016). There are variable estimates of sea-level rise following the Younger Dryas. Stanford et al. (2011) described meltwater pulse 1B as “robustly expressed” as a multimillennial interval of enhanced sea-level rise rates between 11,500 and 8800 calendar yr ago with peak rates of rise of up to 25 m/1000 yr. Using the modeled 11.7 ± 0.1 ka age as the age of the Cross-Hosgri slope lower slope break and 30.3 ± 9.4 m (two standard deviations, 95% confidence limit) as the amount of offset, we calculated a slip rate of 2.6 ± 0.8 mm/yr, which is essentially the same as the previously suggested rate of 2.6 ± 0.9 mm/yr (Johnson et al., 2014). The previous rate was determined using a Monte Carlo simulation in which the age of the lower slope break was assigned a range of 12,000 ± 500 yr, so that possible slip ranged from 20.9 m in 12,500 yr to 39.7 m in 11,500 yr. The wealth of new geochronological data and age modeling in this study (Tables 1–5) reduces the age uncertainty and makes feasible the simpler and more direct statistical methodology approach we used in this study.
Chirp profiles clearly indicate Holocene (since at least ca. 7 ka) activity on the Hosgri fault. Reflection-defined surfaces interpreted as the latest Pleistocene transgressive surface of erosion and an early Holocene wave-erosion surface (unit 1-2 contact) are vertically offset on multiple fault crossings (Figs. 6, 7, and 8). Maximum offsets for the transgressive surface and wave-erosion surface (unit 1-2 contact) are 125 cm and 73 cm. We used ages taken at the base of unit 2 (ca. 7 ka) to constrain the fault activity age, as these sediments preserved the sharp vertical offset along the unit 1-2 contact. In addition, abruptly changing amplitude values were observed in chirp data above the offset unit 1-2 contact on HFC-4 (Fig. 7A), also suggesting fault activity during the deposition of the base of unit 2. The amount of vertical offset is consistently larger for the older surfaces, although the amounts of vertical offset on each surface vary from profile to profile, which is common over short distances for strike-slip faults (e.g., Weldon et al., 1996). This observed offset variation could be due to the Hosgri fault cutting obliquely across the Cross-Hosgri slope, which might potentially produce different apparent displacements.
Advances in Geophysical Imaging and Geochronology
Conducting paleoseismology in the marine environment is difficult, with a significant portion of studies relying on secondary evidence of fault activity (e.g., turbidites; Goldfinger, 2011) or the correlation of offset sedimentary features to approximated sea-level curves to provide timing constraints (e.g., Johnson et al., 2014; Nishenko et al., 2018). However, ultrahigh-resolution subbottom geophysical imaging combined with precise age control from dated sediment cores offer the opportunity to provide primary, near-field evidence of time-constrained coseismic fault offset (e.g., Brothers et al., 2011, 2015; Watt et al., 2016).
Although Johnson et al. (2014) used single-channel sparker data to image and interpret the Cross-Hosgri slope, chirp results from this study show that the sparker data lacked the vertical resolution needed to make accurate interpretations of the post-LGM stratigraphy (Fig. 12A). Furthermore, most offshore chirp studies utilize “envelope” imagery outputted by acquisition systems for the final interpreted image. Such envelope imagery is composed of the original or “real” signal merged with an “imaginary” signal, a 90° phase-shifted version (Hilbert transform) of the real signal (Henkart, 2006). This envelope combination results in only positive amplitudes, loss of polarity information, and a lower-frequency/resolution image than is possible with more advanced processing (Baradello, 2014). In this study, we observed that when processing the “real” or full-waveform chirp data, extra care must be taken in calculating static (swell) corrections and deconvolution, so that the high-frequency polarity-preserved reflections align properly from trace to trace. Doing so preserves the lateral high-resolution content and layering, which can be easily lost with less optimal processing. This is especially important for mapping seismic stratigraphy and fault offsets in coarse-grained, marginally stratified, or extensively bioturbated sediments, where signal penetration can be limited, and strata commonly show minimal impedance contrasts. The shoreface and midshelf deposits of our study provide good examples of difficult-to-image depositional systems where advanced processing can be especially important.
When comparing the envelope to the full-waveform processed chirp data along profile HFC-4, the difference in geological resolvability is stark (Figs. 12B and 12C). For example, on the envelope-processed version of HFC-4, the offset of the shoreface deposit (unit 1) by the Hosgri fault is nearly impossible to resolve (Fig. 12B), whereas this can be confidently interpreted, and offsets can be measured, on the full waveform data (Fig. 12C). Similarly, clearly resolving the transgressive surface of erosion and internal progradational bedding of the shoreface deposit (unit 1) is difficult at best on the envelope-processed data (Fig. 12B). No evidence of offset, nor internal shoreface deposit patterns, is discernible on the sparker profile, which is also marred by short-period multiples (Fig. 12A), which are characteristic of this source (Kluesner et al., 2019). These comparative subbottom profiles show that when attempting to image and study recent or active faulting of Holocene sediments, special care must be taken in selecting the proper sound source, as well as processing the data to the fullest potential.
Although it is common practice during terrestrial paleoseismic work to use the combination of radiocarbon and OSL dating techniques (e.g., Gray et al., 2015; Bennett et al., 2018; DuRoss et al., 2022), this approach (and the associated modeling) is currently in the incipient stages for offshore paleoseismic studies, with this study being the first known application. Other offshore sedimentological studies have looked at sediment ages with both radiocarbon and OSL techniques (e.g., Alappat et al., 2010; Yi et al., 2013; Yang et al., 2015), including a study of sediment transport down the Monterey Canyon system, offshore central California (Stevens et al., 2013). Stevens et al. (2013) attributed differences between OSL and radiocarbon ages to variable sediment transport down the canyon system.
Our study involved shoreface and shelf environments, deposits, and processes, which are different parameters than those examined in the Monterey Canyon system. We found generally good agreement between radiocarbon and OSL CAM model ages, with OSL generally producing slightly older ages. For example, within unit 1, the mean OSL age was ~1000 yr older than the mean radiocarbon age. As discussed earlier, this time gap may reflect the time between last quartz grain sun exposure on land or shallow water and the time when the dated radiocarbon material was deposited and/or died (shells) in deeper water. Another possibility is that the OSL was incompletely reset prior to deposition, with an inherited 1000 yr of apparent age, although we did not see strong evidence for partial bleaching. It is also worth noting that we assumed full saturated water content over the OSL burial period, as the sampled sediments are subaqueous deposits. The generally good correspondence between radiocarbon and OSL dating suggests that the assumptions behind OSL dating are applicable in this environment. This study showed that combining the two techniques can provide additional information to help resolve the depositional history and further strengthen the geochronological model, both of which are crucial when conducting offshore paleoseismic studies.
New data obtained from chirp seismic reflection profiles and shallow sediment cores provided the basis for a reassessment of the slip rate of the Hosgri fault offshore central California. The Hosgri fault cuts the Cross-Hosgri slope, a subtle seafloor lineament that has a complex depositional history that comprises two distinct stratigraphic units. The lower unit (unit 1) overlies the post-LGM transgressive surface of erosion and is interpreted as a shoreface deposit based on seismic facies (offshore-dipping reflections), sediment texture (clean fine sand), sediment infauna, and significant component (~8.4%) of heavy minerals. Radiocarbon and OSL dates and associated Bayesian age modeling of unit 1 are consistent with deposition during the Younger Dryas stadial. The shoreface was abandoned and partly eroded during the subsequent pulse of rapid sea-level rise and transgression that ended ca. 7 ka. Unit 2 consists of fine to very fine sand and silt deposited in a midshelf environment between ca. 7 ka and the present, when the rate of sea-level rise slowed dramatically. Although unit 2 provides a thin (~50–60 cm) cover over the lower slope break of the shoreface, it does not compromise the value of this feature as a piercing point, and the geochronological date from this study combined with the observed seafloor offset suggest a slip rate of 2.6 ± 0.8 mm/yr. This work benefitted significantly from full-waveform processing of chirp data, resulting in significantly higher resolution in coarser-grained strata, which are typically difficult to image. Application of these techniques should have importance in neotectonic studies of shelf and high-energy environments elsewhere. Our novel combination of radiocarbon and OSL dating also provided important insights into depositional history and strengthened the geochronological model, both of which can have similar value in other offshore paleoseismic investigations.
Support for this work was provided by the U.S. Geological Survey Coastal and Marine Hazards and Resources Program and by Pacific Gas and Electric through a Cooperative Research and Development Agreement. We would like to thank Alicia Balster-Gee, George Snyder, Rachel Marcuson, Daniel Powers, Jennifer McKee, Cordell Johnson, and the crew of the M/V Bold Horizon for help in data collection and processing. We thank SeanPaul La Selle, Jillian Maloney, and an anonymous reviewer for their helpful comments and suggestions, which improved the manuscript. This material was also based upon work supported by the U.S. Geological Survey under grant no. G20AS00042 to A.R. Simms. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. government.