Geomorphology 306 (2018) 102–107

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Geomorphology

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Geomorphic origin of Merritt Island-Cape Canaveral, Florida, USA: Apaleodelta of the reversed St. Johns River?

Peter N. AdamsDept. of Geological Sciences, University of Florida, Gainesville, FL 32601, United States

E-mail address: adamsp@ufl.edu.

https://doi.org/10.1016/j.geomorph.2018.01.0050169-555X/© 2018 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 July 2017Received in revised form 5 January 2018Accepted 7 January 2018Available online 11 January 2018

The Merritt Island-Cape Canaveral (MICCSC) sedimentary complex consists of a series of adjacent, non-conformable, beach ridge sets that suggest a multi-phase constructional history, but the feature's geomor-phic and sedimentary origins are not well-understood. In spite of its notable sedimentary volume (surfacearea = 1200 km2), the MICCSC lacks a clear sediment source, or supply mechanism, to explain its presencetoday. Previously published U/Th, radiocarbon and OSL dates indicate that beach ridge deposition was ac-tive during MIS 5 (130–80 ka) on Merritt Island, but has occurred over a shorter, younger time intervalon Cape Canaveral proper (6 ka to present). In this paper, it is proposed that the MICCSC is an abandonedpaleodelta whose fluvial source provided a sediment supply sufficient for coastal progradation. Althoughthe MICCSC, today, does not receive an appreciable sediment supply, the nearly 23,000 km2 drainagebasin of the St. Johns River may well have provided such a sediment supply during MIS 5 times. This low-gradient fluvial system currently empties to the Atlantic Ocean some 200 km north of the MICCSC (nearJacksonville, Florida) but may have flowed southward during the time of MICCSC sedimentary construction,then experienced flow reversal since MIS 5 times. Three possible uplift mechanisms are proposed to explainthe northward down-tilting that may have reversed the flow direction of the St. Johns, abandoning deltaicconstruction of the MICCSC: (1) karst-driven, flexural isostatic uplift in response to carbonate rock dissolu-tion in central Florida, (2) glacio-hydro-isostatic tilting/back-tilting cycles during loading and unloading ofthe Laurentide ice sheet during the Pleistocene, and (3) mantle convection-driven dynamic topographyoperating within southeastern North America since the Pliocene. This example testifies to the sensitivityof low-gradient, low-relief landscapes to various sources of uplift, be they isostatic or otherwise.

© 2018 Elsevier B.V. All rights reserved.

Keywords:Flexural upliftDrainage reversalKarstPaleodeltaCape

1. Introduction

Low-gradient carbonate landscapes in coastal regions are sensitiveto subtle variations in uplift patterns because geomorphic processes inthese settings operate near thresholds associated with sea-level eleva-tion (Passeri et al., 2015). Despite its low topographic relief, the Floridapeninsula exhibits a rich variety of landforms originating from a rangeof geomorphic processes including carbonate sedimentation, longshoresediment transport, karst development, and sea-level oscillation(Fig. 1). The Merritt Island-Cape Canaveral sedimentary complex(MICCSC) is a prominent cuspate foreland that conspicuously interruptsan otherwise continuous, 600 km-long chain of barrier islands along theAtlantic coast of the Florida peninsula. In addition to being the mostrecognizable geomorphic feature of the Florida coast, it is also home tothe primary U.S. spacecraft launch facility, NASA Kennedy Space Centerand the Cape Canaveral Air Force Station, making it a historically iconicsetting as well. Recently, observations of rapid shoreline retreat nearlaunch complex 39 (formerly utilized for Space Shuttle missions and

currently leased to SpaceX) have prompted investigations into themorphodynamics of the cape (Adams et al., 2013) but the short-termstudies have not addressed the long-standing question: why is theMICCSC there?

This coastal cape, and others like it (e.g. Capes Lookout, Fear, andRomain, also on the U.S. Atlantic coast) have prompted researchers toseek explanations for their presence but many of the proposed originsare either insufficient or implausible. The idea that capes form inresponse to eddies shed off from the Gulf Stream current (Abbe, 1895)is undermined by the temporal inconsistency of large-scale turbulentvortices – both their position and intensity. Others have suggested theMICCSC originates from accumulation of longshore drift, by blockageof unidirectional transport, but this explanation falls short becausethere is no mechanism to explain why transport should be interruptedto cause sediment accumulation. Moreover, Stapor and May (1983)have demonstrated, through sediment analysis and numericalmodeling, that intermittent reversals of transport direction createrecirculating convergence cells along the northeastern Florida coast,which conflicts with the idea of unidirectional transport. Recent workby Ashton et al. (2001) has offered the high-angle wave instabilityhypothesis as an explanation for the seaward growth of an initial

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bump in the shoreline, as high-angle waves approaching a curved coastwill create a positive divergence of longshore sediment transportresulting in accretion. Although an elegant explanation rooted inthe dependence of longshore transport rate on wave direction, thishypothesis cannot account for the growth of the MICCSC because ofthe low-angle wave field that typifies the region.

A likely explanation for cape origin at many locations, however, isthe idea that these coastal promontories grow as wave-dominateddeltas of rivers that offer a substantial sediment load at their mouths(Hoyt and Henry, 1971). At first consideration, this hypothesis appearsto fail for the case of the MICCSC because of the absence of a riverwith sufficient drainage basin size to provide an adequate sediment de-livery rate (May, 1972). Although absent today, I explore the possibilitythat such a river was present in the recent geologic past (latePleistocene) and may have provided a sediment load sufficient tobuild a prominent delta (Merritt Island). Published ages of the MICCSCsediments suggest that a prodigious amount of sediment was depositedduring MIS 5 (130–80 ka). The only apparent source capable of provid-ing such a sediment load is the St. Johns River and, although it drains asufficiently large basin, it currently flows northward, away from theMICCSC. If, however, a spatially non-uniform uplift mechanism wereoperating to produce a northward down-tilting of the low-gradient,low-relief, surface of the Florida peninsula, the St. Johns River maywell have reversed its flow direction during the Holocene, causing itto abandon the MICCSC in favor of its modern Atlantic coast outlet200 km to the north (Fig. 1).

In this paper, I present the evidence for this drainage reversal basedon arguments of paleodeltaic abandonment, and offer three possibleexplanations for the uplift mechanisms responsible for the reversal:mantle-driven dynamic topography, glacial isostatic adjustment, andkarst-driven erosional unloading of the upper lithosphere, for whichthere is mounting evidence. Based on the spatial pattern of depositionalages within the MICCSC, I propose that the seaward-most sediments ofthis paleodelta were eroded and redeposited into the southwardmigrating Cape Canaveral proper, after early Holocene flow reversaland that the cape is currently being reshaped by the low-angle wavefield assailing the site today. Lastly, I calculate the range of tilting rates(and uplift rates) that central Florida must have experienced to reversethe (then southward-flowing) St. Johns River and establish the modernriver gradient of its (now northward-flowing) configuration. Althoughspecific to the Florida peninsula, this study provides an example ofhow the geomorphology of a low-gradient, low-relief carbonate coastallandscape is extremely sensitive to subtle changes in uplift, which canoriginate from various processes including mantle-driven dynamic to-pography, glacial isostatic adjustment (GIA), or karst-driven erosionalunloading of the upper lithosphere.

2. Regional geomorphology

The surface of peninsular Florida is dominated by landforms ofmarine origin (shallow carbonate platform and sandy beach deposits)that have been sculpted by geomorphic processes during their intermit-tent subaerial exposure during the late Cenozoic (Schmidt, 1997).Eocene and Oligocene carbonates comprise a Paleogene erosionalsurface that has undergone significant dissolution (Scott, 1997). Theunconformably-overlying Hawthorne group is a siliciclastic cover, innorth and central Florida, that was deposited as sediments shed fromthe southern Appalachians encroached onto the carbonate platformfrom the north during the Miocene (Scott, 1988). This siliciclasticcover thins to the south and to the west, having been largely removedfrom central Florida, with the modern expression of this interfacebeing the Cody Scarp, a slope break of ~30 m in vertical relief(Upchurch, 2007). South of the Cody Scarp, where Paleogene carbon-ates are exposed at the surface, the peninsula has undergone substan-tially more dissolution and karst development than it has north of the

scarp, where the Hawthorne behaves as a protective carapace over theolder carbonate sediments (Fig. 1).

2.1. St. Johns River

The modern St. Johns River flows northward, subparallel to thebarrier islands of Florida's Atlantic coast, from its headwaters atSt. Johns Marsh (9 m amsl) in Brevard County to its outlet at Mayport,approximately 25 kmeast of Jacksonville (Fig. 1). The river's total length(channel length = 496 km, straight-line length = 300 km) is morethan half the length of the entire Florida peninsula and its gradient isamong the lowest on the continent at 0.00002 (2 cm/km). Despite itslow gradient, and distal upstream tidal influence (Lake George), theSt. Johns River has a mean discharge of 400 m3/s at Mayport, due inlarge part to its sizable basin area 22,900 km2 (Fig. 1). The river isthought to have developed by initially collecting runoff and occupyingswales between adjacent Pleistocene beach ridges, then by entrenchingits course through dissolution of subsurface carbonates and channelingenhanced flow volume (White, 1970; Pirkle, 1971; Schmidt, 1997).

2.2. Comparison of MICCSC to modern deltas

Geomorphically-striking are the beach ridge sets that characterizedthe MICCSC (Fig. 2). These features can be constructed by a variety ofprocesses (e.g. swash-built, eolian dune-built), but in all cases theirpresence represents a sediment supply rate that outpaces the rate ofgeneration of accommodation space, resulting in progradation of theshoreline (Tanner, 1995; Otvos, 2000). Previous researchers have recog-nized the non-conformable, or “disjunct”, succession of beach ridge setsthat comprise both Merritt Island and Cape Canaveral proper (White,1970; Brooks, 1972), and noted a remarkably consistent spacing ofindividual beach ridges, on the Cape Canaveral peninsula, of ~110 m(Rink and Forrest, 2005) (Fig. 2).

When viewed in a comparative context to other prominent beachridge settings globally, the MICCSC, and the Merritt Island portion inparticular, bears a striking resemblance to activewavedominated deltas(Anthony, 2015). A nearby example is the Apalachicola River Delta(ARD), in the Florida Panhandle (Tanner, 1988), which drains a basinof 50,505 km2 (19,500 mile2), emptying into the Gulf of Mexico. Barrierislands of the ARD contain beach ridge sets similar to MICCSC and com-parable shoals are also present in the nearshore (Fig. 2). Other examplesof modern deltas with plan-view shapes similar to that of Merritt Islandinclude the Danube delta on the Black Sea coast, which bears similarbeach ridges, and the Ebro delta in the western Mediterranean.

Another piece of evidence suggestive of a deltaic origin for theMICCSC is the position of the upper portion of the Banana River (BananaCreek), which meanders through the middle of Merritt Island west ofLaunch Complex 39 (Fig. 2). Aerial photos that pre-date the construc-tion of the space center reveal a prominent ebb-tidal delta, now obliter-ated by construction of launch pads 39A and 39B, which suggests thepresence of an inlet that connected a substantial fluvial system to theAtlantic Ocean. The inlet was likely the last vestige of the connection be-tween the Banana River (what is left of the paleo St. Johns River) and theAtlantic Ocean.

3. Flexural uplift/Reversal mechanism

The geomorphic evidence assembled builds a case for a deltaicorigin, but the only river of sufficient sediment yield in the area isthe St. Johns River, which empties to the Atlantic Ocean more than200 km north of the MICCSC. Despite the well-established understand-ing that Florida has been tectonically quiescent for most of the Cenozoic(Smith and Lord, 1997; Hine, 2013), there is substantial evidence forvertical deformation of Florida's lithosphere (Winker and Howard,1977; Rowley et al., 2013). Opdyke et al. (1984) proposed the mecha-nism of isostatic uplift due to karst unloading to explain the

Fig. 1. A: Regional digital elevation model (6 arc-sec resolution) of northern and central peninsular Florida identifying locations of St. Johns River (SJR), St. Johns River Catchment Area(SJRC), river headwaters at St. Johns Marsh (SJM, star), Lake George (LG, star), modern river mouth at Mayport (MP, star), Cody Scarp (CS, dashed line), Trail Ridge (TR), Merritt Island(MI), Cape Canaveral (CC), and cape associated shoals (CAS). B: Inset map of Florida displaying location of DEM (box). Dashed line identifies location of CS, which separates area ofMiocene and younger siliciclastic sedimentary cover (SC) on north side scarp from area of intermittently exposed Paleogene Carbonates (EC) on south side.

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Fig. 2.A:MapofMICCSC region constructed from three, separate DEMdata sets.Mainland,Merritt Island, and CapeCanaveral topography derived from1/3 arc-sec (~10m) resolution dataset (Carignan et al., 2014). Note the three different shading ramps, defined in the legend, used to highlight various geomorphic features in submarine vs. subaerial environments. BananaCreek is considered to represent the paleo channel location of the ancestral St. Johns River mouth responsible for delivering sediment to construct the Merritt Island delta prior toabandonment resulting from regional tilting and drainage reversal. B: Inset shows shaded relief topography of beach ridges on Merritt Island, derived from LiDAR data (obtained fromNOAA DigitalCoast website: https://coast.noaa.gov/dataregistry/search/collection).

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anomalously high, young marine ridges and terraces visible in North-central Florida. This idea was elaborated upon by Adams et al. (2010)to estimate ridge/terrace ages corresponding to specific Marine IsotopeStages (MIS) of the Pleistocene, thereby providing a long-term averagedisostatic uplift rate for the region. Most recently, Woo et al. (2017) haverefined the spatial distribution of uplift by using a flexural isostatic cal-culation to match the warped terrace profiles in the region.

As karstification (carbonate rock dissolution) proceeds, the litho-spheric load is lightened and asthenospheric mantle migrates to com-pensate for the mass removed at the surface. Because the area ofkarstification is spatially limited, however, the uplift is not a purely

isostatic response to load removal, but rather a flexural response(Turcotte and Schubert, 2002). Areas on the margins of the carbonatedissolution region are covered by the aforementioned siliciclastic cara-pace – theMiocene Hawthorn Group –which largely shields the under-lying carbonates from chemical dissolution (Scott, 1997). Thesemarginal areas respond to the karst-driven isostatic uplift of the ex-posed carbonate region by producing a flexed (warped) zone that de-cays in curvature away from the carbonate exposure (Winker andHoward, 1977; Adams et al., 2010). This differential uplift patternwould have tilted the platform downward to the north. Given thelow-gradient and low-relief landscape of the Florida peninsula, it

Fig. 3. Hypothesized stage-wise evolution of MICCSC since 130 ka. In all panels, bar identifies position of sampling transect by Burdette (Burdette et al., 2010) yielding MIS 5 OSL ages.A: During MIS 5, the St. Johns River empties its sedimentary load along the central Florida coast building a prominent delta, which will eventually become Merritt Island. B: Sometimeprior to the mid-Holocene, karst-driven isostatic uplift within the central Florida peninsula has driven a drainage reversal, halting sediment delivery to the delta. This allows oceanwaves to erode the outer delta, transport sediment southward (via longshore drift), building the Cape Canaveral peninsula. C: Modern configuration of the MICCSC illustrates thesubstantial southward growth of the cape tip, as well as recent accretion of the False Cape promontory which may be growing seaward in response to redistribution of wave energyflux over the irregular bathymetry of the cape associated shoals (Limber et al., 2017). Dots show sample locations for 6 ka and 4 ka depositional ages of Brooks (1972) and Rink andForrest (Rink and Forrest, 2005), respectively.

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would require less than 1° of tilting to produce flow reversal for a largeriver.

It is important to recognize, however, that isostatic uplift in responseto carbonate rock dissolution is not the only mechanism capable of pro-ducing the tilting hypothesized to reverse the flow of the St. Johns River.Two very plausible alternatives include (1) glacio-hydro-isostatic uplift/subsidence, and (2) mantle convection-driven dynamic topography.Numerous researchers (Peltier, 1998; Lambeck et al., 2003) have recog-nized that the redistribution of loads (ice on the continental landmassduring glacial intervals and water on the continental shelf during inter-glacials) will affect the regional landscape elevations. The maximumdownward deflection of the lithosphere, under the load, decays withdistance from the load to form a region of uplift (the peripheralbulge). Given Florida's location with respect to the Laurentide icesheet during the LGM, it is plausible that lithospheric deflection associ-ated with ice loading may have influenced surface elevations, perhapsbeing responsible for the drainage reversal theorized to occur for theSt. Johns River. Alternatively, it has been shown that mantle flow canproduce topographic anomalies (Forte et al., 2010; Rowley et al.,2013; Moucha and Ruetenik, 2017) also providing a sound explanationfor the differential uplift necessary to produce tilting.

4. Results

4.1. Description of depositional ages

After having described the geomorphology and mechanism of flowreversal, it is useful to examine the ages of the MICCSC sediments tohelp evaluate the uplift/flow reversal hypothesis. Results from fourprominent studies that utilized absolute dating methods for beachridge deposition are reviewed here.

The first attempt to provide absolute ages for deposition of MICCSCsediments was conducted via U-Th dating of mollusk shells (Osmondet al., 1970) providing two groups of depositional ages of 110 ka and30 ka (Fig. 2) for Merritt Island. Although it has been shown thatwhole-shell dating of mollusks by this method can be problematic due

to diagenetic complications (Scholz and Mangini, 2007; Scholz andHoffmann, 2008), these whole-shell ages provide a rough estimate fordeposition age range that can be used to complement other methods.Radiocarbon dating was performed by Brooks (1972) for CapeCanaveral proper, yielding Holocene ages (7.67 to 1.98 ka). OpticallyStimulated Luminescence (OSL) burial ages were determined for CapeCanaveral as ranging from 4.02 ka to 150 ybp (Rink and Forrest, 2005),and examination of the spatial sampling pattern shows that the OSLages conform with the radiocarbon ages published by Brooks (1972).The samples for OSL dating were collected strategically from ridgesequences whose geomorphic pattern indicates unidirectional(southward) progradation of the cape and the ages agree with theprogradational pattern (younger ridges to the south-southeast) yieldinga cape migration rate of approximately 135 m/century. Combiningpush-core OSL dating with ground penetrating radar imaging ofseaward-dipping clinoforms, Burdette et al. (2010) provided evidencefor MIS 5 for buried sediments of Merritt Island, supporting the earlierU-Th ages (Osmond et al., 1970).

Summarizing the absolute dating studies, it is evident that the fluvialsystem supplying sediment to theMerritt Island delta was active duringthe later portion of MIS 5 (80 ka) (Burdette et al., 2010) and possiblystill active up to the mid-Holocene (6 ka), when beach ridge construc-tion began on Cape Canaveral proper (Rink and Forrest, 2005).

4.2. Calculation of tilting rate

Given the modern elevation of the headwaters at St. Johns Marsh(9 m amsl) and assuming a straight line distance from the headwatersto the outlet atMayport (250 km), the regional surface gradient is com-puted to be 4 cm/km, twice that of the stream gradient for the modernSt Johns River, due to meandering and offsets. Using a date range from80 ka to 6 ka to bracket the St. Johns River reversal event (deltaabandonment age) and assuming that most of the tilting is due to upliftwithin the central peninsula, the uplift rate and tilt rate are calculated tobe within the ranges of 0.11 to 1.5 mm/yr and 0.5 to 7 × 10−6°/kyr,respectively.

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5. Discussion and conclusion

The thesis that an uplift gradient exists along a north–south trendline (dip direction) within the Florida peninsula, helps resolve aprevious conundrum – why should siliciclastic deposition (HawthornGroup) have taken place along the “ridge” of the Florida Peninsula(Schmidt, 1997)? It would stand to reason that rivers would bedeflected to either side of this topographic high, making the centralaxis of the peninsula an erosional feature rather than a depositionalbasin. The flexural uplift hypothesis, presented herein, implies that theOcala high and other structural highs were uplifted more recently(since lateMiocene time), making it plausible that therewas accommo-dation space in central Florida where southward shedding sediments,from Appalachian rejuvenation, could encroach upon the platform andaccumulate. In addition, karst-driven flexural uplift (Woo et al., 2017)offers an explanation for the observations of uplift in Florida butwe can-not rule out mantle flow-driven dynamic topography, which has beensuggested for the warping of marine terraces north of 32° latitude(Rowley et al., 2013). It is indeed possible that both karst-driven flexureand dynamic topography influence the uplift within the region.

The sediments in the beach ridge sets of the MICCSC can be used todecipher the rich geomorphic history of the area, as has been done forother beach-ridge plains (Psuty, 1967; Swenson, 2005). Given the agesof Merritt Island (MIS 5) and Cape Canaveral proper (mid Holocene topresent), it would appear that the seaward-most sediments of BananaCreek (ancestral St. Johns River) delta provided source material for thegrowth and southward migration of the Cape Canaveral tip, which con-tinues to be actively translating today. The OSL dates from Rink andForrest (2005) show that the Cape (proper) displays monotonicallyyounger beach ridges southward, that most likely represent reworkedsediments from the original Merritt Island delta, which itselfshows truncated beach ridges consistent with this explanation(Fig. 2). It is likely, therefore, that the delta extended further seawardduring pre-Holocene times, but began witnessing shoreline retreatafter the reversal of the St. Johns River shut off sediment supply some-time after 80 ka. Using previously computed ages (Osmond et al.,1970; Rink and Forrest, 2005; Burdette et al., 2010) and the pattern ofbeach ridge sets, a plausible geomorphic evolution of the MICCSC issummarized in Fig. 3.

This paper assembles geomorphic, geophysical, hydrologic and dat-ing evidence to bring attention to the potential sensitivity of largefluvialsystems to subtle perturbations of the lithosphere's vertical positionwithin low-gradient, low-relief coastal landscapes. Because the Floridapeninsula is nearly planar and horizontal it lives on the tipping pointof drainage rearrangement/reversal as well as all of the geomorphicconsequences associated with that process. This study highlights theprofound geomorphic implications of differential uplift and should aidother researchers working to understand conflicting evidence in otherlow-gradient, low-relief coastal settings.

Acknowledgments

This manuscript benefitted from discussions with John Jaeger,Andrea Dutton, Alessandro Forte, Mark Panning, Han Byul Woo, andScott Miller.

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