Records of Migration and Ebb-Delta Breaching at Historic and Ancient Tidal Inlets along a River-Fed Paraglacial Barrier Island Christopher J. Hein1 (hein@vims.edu), Gregory Fitzsimons2, Duncan FitzGerald3, Andrew Fallon1,4 Department of Physical Sciences, Virginia Institute of Marine Science, College of William and Mary, Gloucester Point, VA, USA; 2Graduate School of Education, University of Massachusetts, Lowell, Lowell, MA, USA; 3Department of Earth and Environment, Boston University, Boston, MA, USA; 4ESS Group, Waltham, MA, USA 1
1. Motivation: Tidal Inlets in the Stratigraphic Record
2. Study Area: Plum Island, Gulf of Maine, USA Figure 1. Location of study site (Plum Island) along the paraglacial (deglaciated ~17 kyr B.P.) coast of western Gulf of Maine, Mass., USA (courtsey E. Hein)
Tidal inlets in the coastal system: • Channels maintained by tidal flow which hydraulically connect backbarrier environments with the coastal ocean • Among the most dynamic and ephemeral features of barrier systems
A
Salisbury Beach
RI
Merrimack River
150 200 North
100 m
South
Freshwater marsh
Core PIG12
50
Foreshore / nearshore (spit)
-2
250 0
100
Inlet fill
150
3600 BP Inlet fill
Inletassociated bar
Estuarine backbarrier
200
Accretionary bar / prebreaching reorientating inlet
TWTT (nanoseconds)
-4
clay silt / very fine sand
0
50
-2
100
-4
150
-6
0
200 25 m
Washover / beach sand Eastward-migrating mid1800s river/inlet channel
-2 -4
East
250 0 50 100 150
-6 -8
Core PIG-26
-10
Estuarine & former river 200 (1700s?) channel
Figure 5. GPR profile across the southern end of The Basin (see Fig. 2). Note eastward-dipping reflections migration of Merrimack River Inlet (MRI) prior to ebb-delta breaching. MRI record is much less complex than PPI (Fig. 3), but illuminates processes responsible for stratigraphy.
6. Traditional Model: Ebb-Delta Breaching
250
ebb flow
Fig. 9)
Figure 8. Conceputal model of ebb-delta breaching at the MRI. Modified from FitzGerald (1993).
1.0 2.0
Lithology Interpretation backbarrier beach & washover sand
medium sand
3.0
coarse sand / fine granules
4.0
coarse granules / pebbles
beach / spit
5.0 6.0
Figure 4. Graphic core log from Geoprobe direct-push core PIG-12 through the base of the PPI (modified from Hein et al., 2012).
250
7.0 8.0 9.0
inlet fill sequence Base of inlet radiocarbon date: 3.6 ka
estuarine / 10.0 backbarrier Mud Sand Gravel
1741
1781
1809
1826
1851
1830
1883
3.0 Salisbury
4.0 5.0 6.0
Merrimack River Inlet channel
7.0
Figure 6. Log of core 8.0 through 1850s MRI. See Box #3 for explantion 9.0 of units. Note similarities to PPI stratgraphy 10.0 in Core PIG-12.
• Erosion of NE dominant Plum Island by longshore transport southerly spillover migrating MRI channels • Inlet becomes hydraulically eroding shoreline inefficient and breaches ebbdelta to create Bar Welding & NE Plum Ebb-Delta Breaching Island (~1851-1914) (~1831-1850) more direct subtidal to interoccupation of tidal marginal (E-W) route spillover shoals channel • Remnant ebb delta migrates onshore & spit extension welds to form landward bar The New migration Basin Point New Point (see details: Deflected Main Ebb Eroding Downdrift Channel (~1800-1825) Shoreline (~1826-1830)
0.0
Lithology Interpretation 0.0 modern soil & washover 1.0 sand
5. The Historic Merrimack River Inlet: Historic Evidence
Core PIG-26 TWTT (nanoseconds) TWTT (nanoseconds)
Elevation (m MSL) Elevation (m MSL)
4. The Historic Merrimack River Inlet: Stratigraphic Evidence
0
-10 2
MRI GPR profile (Fig. 5)
2.0
fine sand
Figure 3. Processed (top) and interpreted (bottom) GPR profile collected across the complex, 3600-year old Paleo-Parker Inlet (PPI) inlet-fill sequence, central Plum Island (see Fig. 3) (modified from Hein et al., 2013). Core logs (see explanation to right) from vibracores, direct-push cores and auger drill cores. Note complex inlet-fill sequence composed of multiple former channels and complex reflection dipping patterns. To the south is a northward-migrating sequence here re-interpreted as originating from re-orientation of PPI prior to ebb-delta breaching (see Box #7). The underlying estuarine / backbarrier unit dates to > 3600 years B.P., prior to migration of Plum Island landward to its current posuition.
West
N
Upland
Core Log Explanation
TWTT (nanoseconds)
Elevation (m MSL) Elevation (m MSL)
0
100
-8
PIG26 (Fig. 6)
Drumlins
Core PIG-12
-2
2
jetties
Marsh 1 km • Island within the longest Intertidal continuous barrier chain Paleo-Parker C sand flats Fig. 2c Inlet GPR profile in the Gulf of Maine Barrier island (Fig. 3) PIG12 Anthropogenic (Fig. 4) • Mixed-energy, tide freshwater marsh dominated (range: 2.7 m) Plum Island Sound • Northern end: Merrimack Parker N River River mouth 5 km N • Influenced by strong Parker River Inlet 1 km northeast winter storms: dominant southerly long- Figure 2. A) Study area overview. B) Northern Plum Island. C) Central shore sediment transport Plum Island and Parker River.
50
-6
The Basin
Explanation
Plum Island
Parker River
Bay of Fundy Study Gulf of Area Maine
Cape Cod 100 km
Merrimack River Inlet
0
-4
NH MA
3. The Ancient Paleo-Parker Tidal Inlet Sequence
0
ME
Fig. 2b
Coastal Setting
Scientific Problem: • Linking inlet-fill sequences to responsible erosionaldepositional processes is largely inferential: based on observational studies at systems without direct relationships to ancient inlet-fills
-6
B
MRI ebbtidal delta
N.B. N.S.
Fig. 2a
Tidal inlets in the stratigraphic record: • Inlet-fill sequences comprise up to 50% of barrier lithosomes in wave-dominated settings (Moslow & Tye, 1985); commonly imaged with ground-penetrating radar (GPR) & sediment cores • Also identified along shallow continental shelves & in rock record
Data Collection
estuarine fluvial Mud Sand Gravel
Merr Riverimack Inlet
Old Point
marginal shoals “Breakers”
ebb-tidal delta Plum former Island Basin
so miguther rati ly on distal shoals
Figure 7. Historic maps (top) and sketches of shorelines changes (bottom) at the MRI over a ca. 150-year period. Data sources: Mitchell (1741); Desbarres (1741); Blunt (1809); Anderson (1826); Anderson (1830); US Coast Survey (1851); USACE (1883).
7. Ancient & Historic Inlets: Insights into the Stratigraphic Record
Time 2: Channel Time 1: Eroding • Updated model of MRI evolution (Fig. Downdrift Shoreline Orientation & Migrating Bar 9): SW & landward-migrating MRI dominant slight eastward longshore reorientation of reoriented seaward in response to transport main ebb channel by wave spillover ebb deflection by a protruding shoreline interaction with channel flow ETD distal bars formed from the onshore welding of wave downdrift ebb-delta sediment ETD action distal more eroding bars effective Plum Island • Ebb-delta breaching more complex (west fork) river/inlet subtidal shoreline channel than traditional model landwarddeposits migrating bar • Re-evaluation needed of details & Time 3: Welding Bar Time 4: Complete Ebb& Channel Deflection complexities observed in stratigraphic Delta Breaching occupation ETD collapse & (GPR, sediment cores) record? of spillover onshore migration channel (example: see PPI sequence, Fig. 3) deflection of channel • Comparison of historic & ancient inlet eastward protoBasin sequences underscores need to carefully remnant new document complex inlet processes reeastward marginal migrating shoals bar welded channel corded in inlet-fill sequences in close to beach deposits comparison with those preserved by Figure 9. Updated model of MRI ebbmodern analogs. delta breaching based on Boxes 4&5.
planned jetties
marginal shoals
marginal shoals distal shoals er
os
ion
New Point Basin
New Point Basin
• Multiple phases of SW inlet migration & erosion of NE Plum Island shoreline • Multiple developments of The Basin & NE fork of island (Old/New Point) • Migration halted by jetty installation (1880s)
8. References
FitzGerald, D.M., 1993. Origin and stability of tidal inlets in Massachusetts, In: Aubrey, D.G., and Giese, G.S. (eds) Formation and Evolution of Multiple Tidal Inlets, Washington DC: AGU, Coastal and Estuarine Studies, 44, pp. 1-61. Hein, C.J.; FitzGerald, D.M.; Carruthers, E.A.; Stone, B.D.; Barnhardt, W.A., and Gontz, A.M., 2012. Refining the model of barrier island formation along a paraglacial coast in the Gulf of Maine, Marine Geology, 307-310, 40-57. Hein, C.J.; FitzGerald, D.M.; Barnhardt, W.A., and Stone, B.D., 2013. Onshore-Offshore Surficial Geologic Map of the Newburyport East and Northern Half of the Ipswich Quadrangles, Massachusetts, Amherst, MA: Mass Geological Survey, Map GM 13-01, map scale 1:24,000, 3 sheets. Moslow, T.F., and Tye, R.S., 1985. Recognition and characterization of Holocene tidal inlet sequences. Marine Geology, 63(1), 129-151.
9. Acknowledgements
This work is funded by US National Science Foundation Award OCE-1325430. We thank M. Morris of the Storm Surge public outreach organization for assistance with historical document research.