Allen M. Gontz

Evolution of the Glaciated Shelf and Coastline of the Northern Gulf of Maine, USA

ABSTRACT The Gulf of Maine (northeast US coast) records shelf evolution since deglaciation ca. 15 ka. Glacial erosion of bedrock left a complex coastline of bays, peninsulas and islands. Till and outwash provided coarse sediment for reworking in littoral systems throughout the Holocene transgression. Glaciomarine mud, the Presumpscot Formation, was an abundant source of fine sediments that were reworked in estuaries, embayments and back-barrier systems. Relative sea-level change was driven by both isostatic and eustatic components. Initial submergence to 70–130 m above present sea level was contemporaneous with marine-based ice-sheet retreat at 15-13 ka. Rapid emergence followed to 60 m below present, during continuing isostatic rebound at 13-11 ka. Finally, submergence and transgression occurred 10.8 ka to present as isostatic rebound was overtaken by eustatic sea-level rise. Reworking during emergence and lowstand brought sand and gravel to the present coast and inner shelf, building paleodeltas at the mouths of the Merrimack River and Kennebec River. Other rivers, such as the Penobscot, drained landscapes with fewer coarse-grained sources, and have primarily mud-filled estuaries, such as Penobscot Bay. Detailed seismic reflection profiling and sidescan sonar mapping provide data for a model of inner-shelf evolution based on principles of sequence stratigraphy. The lower sequence boundary is the unconformity created on top of the Presumpscot Fm. and other glacial sediments during emergence. Lowstand systems tracts are best recognized in paleodeltas. Transgressive systems tracts are thin, but interrupted by parasequences of prograding deltaic and estuarine facies in some estuaries. Significant examples such as the newly discovered Penobscot Paleodelta (8–9 ka, 30 m below present sea-level) may relate to a slowing of relative sea-level rise. Highstand systems tracts formed in the late Holocene as the rate of sea-level rise slowed and the rate of sediment supply allowed stabilization and progradation of barriers and tidal deltas. Preservation potential of these features is controlled by the open coast ravinement unconformity, and by the tidal ravinement unconformity and bluff ravinement unconformity in embayments. Variability in preservation potential results both from paleotopography and differing energy of modern processes.

Coastal Hazard Vulnerability Assessment of Sensitive Historical Sites on Rainsford Island, Boston Harbor, Massachusetts

Abstract It has been well established that numerous coastal areas are threatened by sea-level rise and coastal flooding. Some of these vulnerable lands contain significant archeological sites and cultural resources. The accurate calculation of shoreline rates of change and identification of coastal hazard zones for areas containing cultural resources is crucial for the development of effective coastal zone management strategies that address resource conservation and preservation. This investigation employed geospatial and analytical statistical techniques to conduct a shoreline change study on Rainsford Island occurring from 1944 to 2008. The 4.45-ha island, located in Boston Harbor, Massachusetts, consists of two heavily eroded drumlins connected by a low-lying bar. Past archeological surveys have concluded that Rainsford Island has numerous historical sites and is an area of high prehistoric sensitivity. A recent geophysical survey mapped a Revolutionary War era cemetery on the island. Multiple data sources were integrated, including historical maps, aerial photographs, and airborne laser topographic data for shoreline delineation over various temporal and spatial scales. The Digital Shoreline Analysis System was used to determine rate-of-change statistics and distances, and to identify hotspot areas of erosion and accretion. The results show that the island eroded during the study period at a rate of 0.05 m/y on average, with erosion rates as high as −0.59 m/y. The bar has migrated SE resulting in erosion along the islands northern shoreline. Predictive modeling indicates that 26% of the island would become inundated with 1 m of sea-level rise including the area containing the cemetery.

Seafloor Features and Characteristics of the Black Ledges Area, Penobscot Bay, Maine, USA.

ABSTRACT The Black Ledges, a series of islands, shoals, and ledges in East Penobscot Bay, Maine, was mapped with digital sidescan sonar and shallow marine seismic reflection equipmentA total of 38 km2 of sidescan and 600 km of seismic data was collected during four cruises in 2000–2001. The sidescan sonar reveals a surficial geology dominated by muddy sediments with frequent, patchy outcrops of gravel and minor amounts of bedrock. There are seven large concentrations of pockmarks with populations totaling over 3500 in the areas of muddy sediments. Generally circular, pockmarks range in size from five to 75 meters in diameter and up to eight meters deep. Calculations show over 2 × 106 m3 of muddy sediment and pore water were removed from the system during pockmark formation. Seismic data reveal a simple stratigraphy of modern mud overlying late Pleistocene glaciomarine sediment, till and Paleozoic bedrock. Seismic data indicate areas of gas-rich sediments and gas-enhanced reflectors in close association with pockmarks, suggesting methane seepage as a cause of pockmark formation. Pockmarks are alsorecognized in areas lacking evidence of subsurface methane accumulations adding further validity to the late stage of development for the field. Elliptical pockmarks, found in nearly 40 m of water, show modification by currents and degradation of the pockmark form. This suggests depletion of methane and a late stage of development. A more intensive investigation of the area, including coring and high-resolution geophysics is currently in progress.

Paleodeltas and Preservation Potential on a Paraglacial Coast — Evolution of Eastern Penobscot Bay, Maine

The bedrock framework of the northern Gulf of Maine coast, USA (Fig. 1), controls the geometry of headlands and embayments (Shipp et al., 1985, 1987; Kelley, 1987). Quaternary continental glaciers sculpted this paraglacial coast, culminating in the latest Wisconsinan Laurentide Ice Sheet, which reached its maximum extent in the region 20–22 ka (Hughes et al., 1985). This ice sheet was marine-based in much of the Gulf of Maine 20–15 ka (Schnitker et al., 2001) and during later stages of retreat through the Maine coastal lowlands (Stuiver and Borns, 1975; Dorion et al., 2001). Sediments of a wide variety of (Thompson and Borns, 1985) were deposited duringeglacial retreat, interpreted in a sequence-stratigraphic model by Belknap and Shipp (1991) and Barnhardt et al. (1997). Sediment sources to the evolving Holocene coast included reworking from glacial and glaciomarine outcrops, as well as limited fluvial inputs.

Stratigraphic Architecture of a Regressive Strand Plain, Flinders Beach, North Stradbroke Island, Queensland, Australia

ABSTRACT Gontz, A.M.; Moss, P.T., and Wagenknecht, E.K., 2014. Stratigraphic architecture of a regressive strand plain, Flinders Beach, North Stradbroke Island, Queensland, Australia. Regressive beaches are commonly composed of extensive and complex beach ridge systems. The physical and stratigraphic relationships of the beach ridges can help decipher the interactions between sea-level changes, variation in storm periodicity and intensity, changes in current directions, and alterations in sediment sources. Similar systems have been identified on the SE coast of Queensland and the spatial and temporal relationships between the various depositional environments are poorly understood. A small-strand plain system was identified on the NE coast of North Stradbroke Island, Queensland, Australia, and targeted for study with ground-penetrating radar (GPR). A series of GPR transects were acquired to elucidate the subsurface architecture of the system and to establish a geologic and geomorphic framework for the system through the application of stratigraphic analysis and decipher the spatial relationship. The GPR survey was concentrated in the center of the system and acquired approximately 3 km of 250-MHz data. The survey shows the stratigraphic relationship between modern sand dunes, beach ridge sequences, wetlands, and the much older Pleistocene sand dunes and identifies a potential Holocene storm record. We established a stratigraphic link between environments and propose a model driven by sea-level change, which proposes beach-ridge development during the early/mid-Holocene regression.

The Duxbury Sunken Forest—Constraints for Local, Late Holocene Environmental Changes Resulting from Marine Transgression, Duxbury Bay, Eastern Massachusetts, U.S.A.

ABSTRACT Gontz, A.M.; Maio, C.V., and Rueda, L., 2013. The Duxbury sunken forest—constraints for local, late Holocene environmental changes resulting from marine transgression, Duxbury Bay, Eastern Massachusetts, U.S.A. The present marine transgression has forced geological and ecological zones vertically higher and landward since the late Pleistocene. A recent investigation in Duxbury Bay, Massachusetts, identified 18 Juniperus virginiana tree stumps emergent on an intertidal flat immediately seaward of a small marsh and pond situated between two eroding drumlins. The position of each stump was mapped with global positioning system (GPS), and its elevation with respect to mean lower low water was surveyed. Samples were selected from four stumps with elevations ranging from 2.03 and 0.75 m above mean lower low water for radiocarbon dating. The samples returned calibrated ages between 2219 ± 94 and 2867 ± 79 cal YBP, with the topographically highest sample returning the youngest date. Stump positions suggest a landscape gradient of 1.4 mm/yr between 2000 and 3000 cal YBP. The results are comparable with high-resolution studies of sea level in eastern Massachusetts for the same time period. Comparison of the youngest paleostumps with modern living trees suggests a dramatic change in the landscape gradient, an increase to 1.8 mm/yr. While this is contrary to sea-level studies nearby, it may represent an increase in the energetics of Duxbury Bay and resultant coastal erosion as the bay floods. The site can be used to put the impacts of changing sea-level rates into a landscape evolution framework.

Subsurface Evidence of Storm-Driven Breaching along a Transgressing Barrier System, Cape Cod, U.S.A.

ABSTRACT Maio, C.V.; Gontz, A.M.; Sullivan, R.M.; Madsen, S.M.; Weidman, C.R., and Donnelly, J.P., 2016. Subsurface evidence of storm-driven breaching along a transgressing barrier system, Cape Cod, U.S.A. Relict and historic tidal channels buried within coastal barriers provide a geologic signature of environmental change, thus enhancing our understanding of how barrier systems respond to extreme storm events. Earliest maps from 1846 depict three inlets along the Waquoit Bay barrier system located on Cape Cod, Massachusetts. These channels were not depicted on maps after 1846, and we lack any information pertaining to them before 1846. The principle objective of this study was to identify the location and map the internal geometry and channel-fill configuration of the buried inlet structures using geophysical and sedimentological data acquisition methods. This was done by collecting 6.2 km of shore-parallel ground-penetrating radar data and five sediment cores ranging in depth from 4 to 5 m. The sediment cores allowed for the ground truthing of the ground-penetrating radar data and provided six samples for radiocarbon dating. The 13 paleochannels identified ranged in depths from 1.3 to 3.7 m below the present beach surface. These appeared in the radar imagery as broad U-shaped cut-and-fill features incised into adjacent barrier facies. The 13 paleochannels composed 24% of the barrier lithosome totaling 704 m in length. Individual channels were primarily less than 65 m in length and between 2.5 and 1.3 m in depth, although an additional 275-m-wide, 3.7-m-deep channel sequence was imaged and likely represents a major and long-lived paleochannel. The results will contribute toward deciphering the evolution of the Waquoit system and identify areas vulnerable to storm-driven coastal change.

Ground Penetrating Radar Observations of Present and Former Coastal Environments, Great Sandy National Park, Queensland, Australia – Focus on Moon Point, Fraser Island

ABSTRACT Gontz, A.M; McCallum, A.B., Moss, P.T., and Shulmeister, J., 2016. Ground penetrating radar observations of present and former coastal environments, Great Sandy National Park, Queensland, Australia. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 730–734. Coconut Creek (Florida), ISSN 0749-0208. The worlds large sand islands and mainland sand masses dominate the coastal system of southeastern Queensland with dunes exceeding 240 m in height. Previous work using sediment cores and TL/OSL dating has estimated the age of the dunes at 730,000 years. This system, the worlds largest downdrift sand system, has Fraser Island and the Coolala Sand Mass as the two northern-most members. Combined, these form the Fraser Island UNESCO World Heritage Site and the Great Sandy National Park. Between 2012 and 2015, a series of high-resolution, reconnaissance-level GPR lines were acquired on Fraser Island and on the northern end of the Coolala Sand Mass; totalling over 75 km. The lines provide insight into sea-level fluctuations that drive large-scale coastal evolution and reveal the presence of past shoreline complexes that are proposed to be associated with the mid Holocene highstand and the previous interglacial. The generalized sequence is interpreted as a variable thickness of aeolian sand over beach deposits with a transgressive unconformity at the base of the beach deposits, which is interpreted as the Mid Holocene-to-present system. The Mid Holocene system overlies an older system that has variable preservation and is composed of beach deposits with a transgressive unconformity at the base. In places, portions of the sequence are lightly consolidated with organic material, locally known as “coffee rock”. The coastal sequences hold the potential to host an archive of storm, sea level and sediment supply records over at least two interglacial periods.

THE ROLE OF BACKBARRIER INFILLING IN THE FORMATION OF BARRIER ISLAND SYSTEMS

Barrier islands develop through a variety of processes, including spit accretion, barrier elongation, and inlet filling. New geophysical and sedimentological data provide a means of documenting the presence of a paleoinlet within a barrier lithosome in the western Gulf of Maine, illuminating the process of backbarrier infilling and its effect on barrier and tidal inlet morphodynamics. The transport of sediment into the backbarrier through tidal inlets as well as sediment contribution from nearby rivers led to bay infilling, formation of tidal flats and marshes, and a vast reduction in the bay tidal prism. Using existing marsh stratigraphy and high resolution imaging of a paleo inlet, this study investigates the effects of this diminishing tidal prism and inlet closure process. Chronostratigraphic reconstructions and digital backstripping of the backbarrier explain rates and timing of infilling and eventual conversion of an open water lagoon to the modern high marsh and tidal creek system.

Controls on Estuarine Sediment Dynamics in Merrymeeting Bay, Kennebec River Estuary, Maine, U.S.A.

Over the past several decades, estuaries have earned a reputation as sediment sinks through the theoretical and empirical works of many scientists (e.g. Postma, 1967; Pritchard, 1967; Meade, 1969, 1972, 1982; Biggs, 1970; Biggs and Howell, 1984; Schubel, 1984; Knebel, 1989; Dalrymple et al., 1990). These studies have documented the combined roles of sediment influx rates, sea-level rise, climate, and estuarine circulation as the dominant controls on estuarine infilling (Schubel, 1984). However, aspects of most of these models (e.g. distance-velocity asymmetry and settling lag and scour lag) only consider the movement of fine-grained sediments (<100 µm) capable of suspension or transport-limited systems in which estuarine sediment supply is greater than the transport capacity (Milliman and Meade, 1983). Much less is known about the dynamics (i.e. estuarine processes and time scales responsible for sediment fluxes) within fluvial-estuarine transition zones with respect to bedload sediment transport (Milliman and Meade, 1983)