Available online at www.sciencedirect.com R
Marine Geology 200 (2003) 291^306 www.elsevier.com/locate/margeo
Hardbottom development and signi¢cance to the sediment-starved west-central Florida inner continental shelf Stephen P. Obrochta a , David S. Duncan b , Gregg R. Brooks c; b
a College of Marine Science, University of South Florida, 140 7th Avenue South, St. Petersburg, FL 33701, USA Department of Environmental Science and Policy, University of South Florida, 4202 East Fowler Avenue, Tampa, FL 33620, USA c Department of Marine Science, Eckerd College, 4200 54th Avenue South, St. Petersburg, FL 33711, USA
Accepted 1 June 2003
Abstract Hardbottoms are sequence boundaries and condensed sections that offer clues for the interpretation of the incomplete record of Tertiary continental shelf evolution. Seaward of 5 km, 50% of the inner west-central Florida shelf seafloor is flat hardbottom. These lithified surfaces are punctuated by shorefacing, scarped hardbottoms that trend shore-parallel (330‡^0‡) and vary in relief (up to 4 m). Scarped hardbottoms are the only natural relief on the inner shelf and support a diverse benthic community, the activities of which erode the outcrops, producing undercuts in excess of 1 m. Outcropping hardbottom strata are comprised of distinct, phosphate-rich, mixed carbonate^ siliciclastic lithofacies, that range in age from Miocene to Quaternary. Miocene units are dolomite-rich and mark the upper surface of the inner shelf bedrock (Hawthorn Group). Dolomite within these beds (silt-sized, cloudy centered rhombs) fall into two age groups, correlating with highstands at 15 and 5 Ma. This lithofacies is consistent with models that indicate an increased flux of organic matter ^ resulting from topographically induced upwelling ^ promoting dolomitization during early burial diagenesis in the sulfate-reduction zone. Quaternary units are calciterich and perched atop the shelf bedrock. Samples of these units record a complex diagenetic history and multiple sealevel fluctuations. Based on evidence of primary marine cementation, they are interpreted to be hardground (nondeposition) surfaces, forming as a function of sediment starvation and minimal sediment movement. Decreased highstand magnitude or duration may have resulted in the absence of a significant organic component to Quaternary hardbottoms, which, in turn, may prevent subsequent dolomitization. These outcrops are a potential source for sediments to the inner shelf, not only as habitat for biological sediment production, but also through their destruction. The undercut, shorefacing, scarped hardbottom morphology displayed by west-central Florida hardbottoms is indicative of bio-erosion. Preliminary studies indicate a potential mass of 0.04 kg m32 yr31 of siliciclastic sediment is released to the inner shelf. 7 2003 Elsevier B.V. All rights reserved. Keywords: carbonate diagenesis; hardbottoms; carbonate petrology; dolomitization; shallow water carbonates
* Corresponding author. E-mail addresses:
[email protected] (S.P. Obrochta),
[email protected] (D.S. Duncan),
[email protected] (G.R. Brooks).
1. Introduction Hardbottoms, or lithi¢ed sea£oor, are common in carbonate and siliciclastic shallow marine set-
0025-3227 / 03 / $ ^ see front matter 7 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0025-3227(03)00188-9
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tings worldwide, but these features have been poorly documented. Along the west-central Florida coast, at least 50% of the inner shelf sea£oor seaward of 5 km consists of hardbottom (Locker et al., 2003). Extensive systems of scarped hardbottom constitute the only areas of natural relief (up to 4 m), providing important marine habitat, and therefore processes controlling their formation and distribution are of interest biologically. In addition, the lithology and stratigraphy of these outcrops provide evidence for interpreting the incomplete geologic record of Tertiary continental-shelf evolution. Extensive research of hardbottoms in Onslow Bay, North Carolina by Riggs et al. (1996, 1998) has pointed out the importance of hardbottoms as sequence boundaries and condensed sections often constituting the preserved sedimentary record of highstand and lowstand sea-level events. The hardbottoms of Onslow Bay occur on a highenergy continental shelf, where lithology plays a key role in controlling their morphology (Riggs et al., 1998). In addition, bio-erosion of these features, as well as production of the associated benthos, contributes signi¢cant amounts of ‘new’ sediment to the inner shelf system (Riggs et al., 1998). West-central Florida hardbottoms di¡er from those of Onslow Bay in lithology, stratigraphy, and relief. Currently, the age and means of formation of west Florida hardbottoms are not well understood. Observations provide two simple models of formation ; namely, the scarped hardbottoms are erosional features cut into the exposed underlying bedrock, or these features represent a younger outcrop formed by processes not necessarily related to those of the bedrock. Initial studies have shown that these hardbottoms do contribute sediments to the shelf,
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but the total volume is unknown (Winston et al., 1968; Riggs and O’Connor, 1974; Berman, 1998). The objectives of this study are to determine the age, lithology, diagenetic history, dynamics, and origin of west-central Florida hardbottoms, as well as their signi¢cance to the sedimentary development of the continental shelf. 1.1. Geologic setting The west-central Florida continental shelf is a low-energy, sediment-starved, gently sloping surface comprised of a mixed carbonate^siliciclastic sediment veneer overlying a lithi¢ed carbonate surface (Brooks et al., 2003). At the mouth of Tampa Bay, near Egmont Key, this lithi¢ed surface has been correlated with the dolomite-rich Arcadia Formation of the Hawthorn Group (Duncan, 1993; Duncan et al., 2003). However, the o¡shore extent of this correlation is unknown. Dolomite, calcite, quartz, palygorskite, and francolite are the primary minerals present within the Hawthorn Group, interpreted as a vast, low-energy, shallow marine deposit (Scott, 1988). A mixed carbonate^siliciclastic sediment veneer with varying amounts of phosphorite grains overlies the lithi¢ed surface (Doyle and Sparks, 1980; Brooks et al., 2003). The carbonate fraction, consisting of coarse sand to granule-sized shell fragments, has been interpreted to be marine sediment deposited during the current Holocene sea-level transgression (Brooks et al., 2003). The siliciclastic fraction is predominantly comprised of ¢ne sand-sized, subangular to subrounded quartz. Siliciclastic sediments are lacking in the modern river systems entering the Gulf of Mexico along west-central Florida, so the siliciclastics found on the shelf are thought to be reworked from
Fig. 1. Map of hardbottoms and sediment vibracore locations showing bathymetry and landsat image of west-central Florida. Letters refer to map locations (Table 1). Sediment vibracores penetrated through the sediment veneer and were refused by lithi¢ed surfaces, retaining rock fragments in the bottoms of the barrels. These fragments were compared to samples of scarped hardbottoms. Filled and open triangles refer to dolomitic and calcitic hardbottoms, respectively. Filled and open circles refer to dolomitic and calcitic rock fragments, respectively. Map adapted from Gelfenbaum and Guy (2000).
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the erosion of pre-existing stratigraphic units (Brooks et al., 2003).
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3. Results 3.1. Observations
2. Methods Di¡erential Global Positioning System (DGPS) was used for navigation, and the bathymetric con¢guration of each outcrop was determined using a precision depth recorder. SCUBA divers sampled outcropping beds of scarped hardbottoms throughout the inner west-central Florida continental shelf. These samples were compared to rock fragments from the bottoms of sediment vibracores that penetrated to bedrock (Fig. 1). Each sample was thin-sectioned and stained with alizarin red to di¡erentiate dolomite from calcium carbonate. Percent carbonate (% CO3 ) analysis was performed on crushed samples with 10% HCl. A settling tube (Gibbs, 1974) was used to generate grain-size distributions for the sand fraction ( s 63 Wm) of the insoluble residue. A Scintag XDS 2000 X-ray di¡ractometer (Cu KK radiation) was used to determine the bulk mineralogy of selected samples. Prior to X-ray di¡raction (XRD) analysis, the outer layer of heavily encrusted samples was removed with dilute, 10% HCl to allow the mineralogy of the framework to be determined. Strontium isotope ratios were determined by Geochron Laboratories of Cambridge, MA. The bulk 87 Sr/86 Sr ratios of six texturally homogeneous samples were measured, and one dolomite sample was disaggregated so the 87 Sr/86 Sr ratio of the dolomite and phosphorite could be separately determined. Ages were calculated from the 87 Sr/86 Sr seawater curve and regression equations of Hodell et al. (1991) and Martin et al. (1999).
Scarped hardbottoms trend shore-parallel (330‡^0‡) and vary in relief (up to 4 m) (Fig. 2). Flat hardbottoms constitute their upper bounding surfaces. The sediment veneer extends to the scarp. These outcrops contain diverse benthic communities that appear to be concentrated on the shore-facing (lee) side of scarped hardbottoms. Halimeda sp. meadows cover the upper £at hardbottoms, proximal to the scarp. Red calcareous algae, boring mollusks (Lithophaga sp.), boring sponges (Cliona sp.), and echinoderms occupy both upper £at and scarped surfaces. Gorgonians and Millepora sp. are also commonly observed. Dawes and Lawrence (1989) have described a diverse benthic £ora, while Winston et al. (1968) provide a more complete list of invertebrate organisms that make up these diverse hardbottom communities. Undercutting of the scarp, in some places in excess of 1 m, may be the result of bio-erosion much like that described by Riggs et al. (1996, 1998) in Onslow Bay. Divers commonly observe a proximal rubble ramp/¢eld created as overhanging blocks have broken o¡ and deteriorated. 3.2. Lithology Interpretations based on thin-section analysis indicate lithologic complexity. Both hardbottom samples and vibracore rock fragments exhibit a mixed carbonate^siliciclastic mineralogy and crystalline (Fig. 3A,D) or grain-supported (Fig. 3C) textures. The non-carbonate fraction consists primarily of moderately well sorted, subangular to
Fig. 2. (A) Photographs of scarped hardbottoms showing lithi¢ed, encrusted surfaces. Undercutting may be on the order of 1^2 m while relief may be up to 4 m. Note the thick Halimeda sp., occupying the top of the outcrops, coralline red algae and sponges encrusting the sides. U-FHB and SHB stand for upper £at hardbottom and scarped hardbottom, respectively. (B) Sun-illuminated multibeam bathymetry map of the 12P Ledge (Site F) adapted from Farmer et al. (1999) and Naar et al. (1999). Sun illumination is from the northeast. The outcrop generally trends north-northwest (330‡^0‡) with the main scarp facing northeast (shoreward). Image has not been post-processed and is available online at http://www.marine.usf.edu/geology/moontan/c£oor/ meg/index.html.
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Fig. 3. Photomicrographs of various samples. (A) Crystalline-textured low-Mg calcite spar with relict shell texture, which is the product of aggrading neomophism (meteoric phreatic diagenesis). Thin section was stained with alizarin red (HB-96-25; PPL). (B) Relict textures of an isopachous, blocky to acicular cement (10 WmU40 Wm) lining the substrate of pores ¢lled with drusy low-Mg calcite cement. Original cement was probably marine phreatic high-Mg calcite. Qtz. = quartz grain (HB-96-24; PPL). (C) Grain-supported sample, mainly consisting of sub-angular to sub-rounded ¢ne sand-sized quartz (HB-96-22; XN). (D) Siltsized, cloudy-centered, zoned dolomite rhombs, common in Hawthorn Group strata (HB-96-3; XN). (E) Pore-¢lling, low-Mg calcite cement with textures of earlier marine phreatic cement. Micritized skeletal grain is a miliolid foraminifer (HB-96-21; PPL). (F) Mollusk fragment which has been replaced by sparry, low-Mg calcite. Met. = meteoric low-Mg calcite, and Mar. = relict textures of marine cement (HB-96-27; XN).
subrounded, ¢ne sand-sized quartz grains (Fig. 3C, E,F). The size and shape of quartz grains are consistent with those of the inner shelf sediment veneer (Brooks et al., 2003). Low-Mg calcite, ara-
gonite, and dolomite are present in both sea£oor outcrops and vibracore rock fragments, while high-Mg calcite was only observed in vibracore rock fragments. Dolomite is most common as
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Fig. 4. Typical XRD trace of (A) calcite-rich hardbottoms, (B) dolomite-rich hardbottoms, and (C) blue-green clay. LMC and Q stand for low-Mg calcite and quartz, respectively.
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very ¢ne sand to silt-sized, cloudy-centered, zoned rhombs (Fig. 3D). Low-Mg calcite exists as neomorphic spar (Fig. 3A,F) and pore-¢lling cement (Fig. 3B,E). Aragonite probably comprises micritized skeletal grains as well as lime mud (Fig. 3C,E,F). Unidenti¢ed molluscan fragments comprise most of the skeletal grains. Micritized, shallow-marine foraminifers, such as miliolids (Fig. 3E), Cyclorbiculina, and Archaias are locally abundant in the micritic matrix. XRD results con¢rm petrographic interpretations (Fig. 4). Rounded, ¢ne sand-sized, pelloidial phosphorite grains (Fig. 3B,C,E) are francolite (carbonate £uorapatite). Palygorskite is an important clay constituent. Table 1 presents a summary of mineralogical data from (1) hardbottoms and (2) rock fragments from the bottoms of vibracores that encountered the underlying limestone. 3.3. Age The hardbottoms fall into three age groups ^ middle Miocene, late Miocene, and Quaternary ^ based on strontium chronostratigraphy (Table 1). All dolomite samples yielded Miocene ages (middle and late), while all calcite samples are Quaternary. The middle Miocene age is consistent with ages reported for the Arcadia Formation of the Hawthorn Group (Brewster-Wingard et al., 1997; Guertin et al., 1999). The late Miocene age falls within the Hawthorn Group, but it is unclear whether this represents Arcadia, Peace River, or some other unde¢ned unit. The phosphatic grains also give a middle Miocene age, and have been incorporated into the hardbottoms during their formation. Compton et al. (1994) describe the age of phosphate formation on the Florida Platform and the age for these grains is consistent with their ¢ndings.
4. Discussion 4.1. Lithofacies and architecture Based on dominant mineralogy, three distinct lithofacies have been de¢ned from hardbottom samples on the shelf, the dolomite and quartz
(DQ) lithofacies, the calcite and quartz (CQ) lithofacies, and the blue-green clay (BGC) lithofacies. Both the DQ and CQ lithofacies are also present in the base of sediment vibracores as rock fragments. Additionally, within sediment vibracores, the BGC lithofacies overlies DQ rock fragments (Brooks et al., 1999, 2003). Each lithofacies contains francolite (phosphate) varying from trace to 10% (Table 1). Lithofacies de¢ne two hardbottom types, calcite-rich and dolomite-rich. The BGC lithofacies occurs at the base of several calciterich outcrops. An idealized lithofacies succession is presented for scarped hardbottoms (Fig. 5), in which the dolomite represents the upper surface of the bedrock (top of Hawthorn Group). Overlying this is the blue-green clay and at the top of the column is Quaternary calcite. 4.1.1. Dolomite and quartz (DQ) lithofacies The DQ lithofacies is interpreted to be a lowenergy deposit within the Miocene Hawthorn Group. It is V70^95% silt-sized rhombohedral dolomite with quartz and minor amounts of palygorskite and francolite, which is consistent with the mineralogy of the Hawthorn Group (Scott, 1988) (Fig. 3D). The texture of the DQ lithofacies indicates deposition in a low-energy environment; rhombohedral dolomite commonly forms from ¢ner-grained, precursor particles (Tucker and Wright, 1990). Compton et al. (1994) proposed that Hawthorn dolomite formed during early burial diagenesis in the marine environment. Topographically induced upwelling during highstands increased productivity and the subsequent £ux of organic matter to the sea£oor. Degradation of this organic material in the sulfate-reduction zone promoted dolomitization. These conditions are also conducive to phosphogenesis. As a result, dolomite and phosphorite within Hawthorn sediments may form contemporaneously. If indeed diagenesis took place in the marine environment, the 87 Sr/86 Sr ratios of the dolomite within the DQ lithofacies probably re£ect the timing of dolomitization (Compton et al., 1994). Dolomite-rich hardbottoms crop out as low-relief ( 6 2 m) features comprised entirely of the DQ lithofacies. The 87 Sr/86 Sr ratio-derived ages from
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Table 1 Mineralogy and age data for the samples of this study Sample number
Location name
Map location
Type
Relative position
Water depth
Carbonate fraction
Quartz
PO4
(m)
Bulk % Mineralogy
approx. % XRD
approx. %
Garden Garden
A A
hardbottom hardbottom
top base
21 21
28 74
dolomite/lmc dolomite
65 24
HB-96-3
Garden
A
hardbottom
base
21
89
dolomite
HB-96-4
Xmas Tree
H
hardbottom
base
23
91
HB-96-5 HB-96-6 VC-96-7 VC-96-8 VC-96-9 VC-96-10 VC-96-11 VC-96-12 VC-96-13 VC-96-14 VC-96-15 VC-96-16 HB-96-17 HB-96-18 HB-96-19 HB-96-20 HB-96-21 HB-96-22 HB-96-23
Hill B Hill A USGS 95-39 USGS 95-39 COE 94-1 COE 94-1 COE 94-12 COE 94-10 COE 94-3 USGS 95-54 USGS 95-110 USGS 95-112 Scott’s Ledge Xmas Tree Xmas Tree Xmas Tree Holding Grounds Holding Grounds Holding Grounds
C C P P Q Q R S T U V W L H H H D D D
hardbottom hardbottom lithoclast lithoclast lithoclast lithoclast lithoclast lithoclast lithoclast lithoclast lithoclast lithoclast hardbottom hardbottom hardbottom hardbottom hardbottom hardbottom hardbottom
surface surface 210 cm 180 cm 295 cm 312 cm 158 cm 41 cm 154 cm 335 cm 62 cm 50 cm surface top top rubble rubble rubble base
12 12 5 5 9 9 6 6 7 5 10 17 9 22 22 22 16 16 16
72 81 65 68 76 79 98 36 67 45 94 82 77 96 93 83 79 72 0
HB-96-24 HB-96-25 HB-96-26 HB-96-27 HB-96-28 HB-96-29 HB-96-30 HB-96-31 HB-96-32 HB-96-33
Holding Grounds Holding Grounds South Jack South Jack South Jack South Jack South Jack 12P Ledge 12P Ledge 12P Ledge
D D E E E E E F F F
hardbottom hardbottom hardbottom hardbottom hardbottom hardbottom hardbottom hardbottom hardbottom hardbottom
rubble top upper A upper B mid ledge rubble A rubble B top top mid
16 16 15 15 15 15 15 18 18 18
69 91 82 77 68 100 76 79 68 90
HB-96-34 HB-96-35 HB-96-44
12P Ledge Venice Bch Aquarium
F G B
hardbottom hardbottom hardbottom
clay base surface base
18 5 23
41 56 78
87
5 5
DQ DQ
10
5
DQ
dolomite
6
5
DQ
dolomite dolomite dolomite dolomite
23 19 33 30 25 20 1 60 28 52 4 17 25 3 4 9 21 28 0
7 1 3 3 3 3 1 3 5 3 2 1 1 2 3 9 trace trace 0
DQ DQ DQ DQ CQ CQ DQ DQ DQ CQ DQ CQ DQ DQ DQ DQ CQ CQ BGC
Y Y Y
trace trace trace trace 2 0 1 2 trace 1
CQ CQ CQ CQ CQ coral CQ CQ CQ CQ
Y Y
6 18 2
BGC CQ DQ
lmc/hmc/ara dolomite dolomite dolomite lmc dolomite dolomite dolomite dolomite dolomite dolomite lmc/ara
lmc/ara lmc lmc
lmc (ara?) lmc/ara lmc
31 9 18 23 19 0 23 19 32 10
dolomite lmc dolomite
12 26 10
(coral frag.)
Y Y
Y Y Y Y
Y
Y
Y Y Y Y Y Y
Age Sr/86 Sr
dolomite and phosphate of HB-96-3 0.708753 Miocene (14.6 T 1.36) 0.708647 Miocene (17.4 T 0.74)
0.710270 higher than modern S.W.
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HB-96-1 HB-96-2
Lithofacies
0.709191 Quaternary/ Holocene
0.708989 Miocene (4.94 T 0.50)
299
Sr/86 Sr ratios used Hodell et al. (1991). The
CQ 56 13 top hardbottom M F-2 HB-97-60
The Late Miocene 87 Sr/86 Sr ratios were converted to ages using regression equations of Martin et al. (1999). All other abbreviations lmc, hmc, and ara, stand for low-Mg calcite, high-Mg calcite, and aragonite, respectively.
Y 20 lmc/ara 78 22 13 base base hardbottom hardbottom H-1 F-2 HB-97-53 HB-97-59
K M
87
these outcrops indicate two separate episodes of dolomitization during the middle Miocene (Site A) and late Miocene (Sites B, I). Phosphorite grains extracted from a hardbottom sample (HB-96-3 ; Site A) indicate they formed around 3 Ma earlier than the dolomite. The results of Compton et al. (1994) show 87 Sr/86 Sr ratio of francolite grains incorporated into Hawthorn Group rocks are nearly contemporaneous in age. The 3 Ma di¡erence is within their variability, however, and is consistent with organogenic dolomitization of these rocks.
87
CQ CQ 5
0.709180 Quaternary/ Holocene Y
DQ DQ CQ 5 5 trace Dome H-2 H-2 HB-97-46 HB-97-47 HB-97-50
I J J
hardbottom hardbottom hardbottom
top top base
22 18 18
79
lmc/hmc/dol N/D lmc/ara
10 25 15
Y
0.709015 Miocene (4.75 T 0.50) DQ 2 Y 10 dolomite 66 22 base hardbottom Dome HB-97-45
I
Sr/86 Sr 87
approx. % Bulk % Mineralogy (m)
approx. % XRD
Lithofacies PO4 Quartz Carbonate fraction Water depth Relative position Type Map location Location name Sample number
Table 1 (Continued).
0.709189 Quaternary/ Holocene
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Age
300
4.1.2. Calcite and quartz (CQ) lithofacies The texture and mineralogy of the CQ lithofacies, although highly variable (Fig. 3A^C,E,F), exhibits di¡ering lithologies as a function of stratigraphic position within outcrops. Basal beds are crystalline-textured and comprised entirely of low-Mg calcite and quartz (Fig. 3A). Overlying beds have a grain-supported texture and contain low-Mg calcite and aragonite (Fig. 3B,C,E,F). While all beds within the CQ lithofacies contain diagenetic textures indicative of subaerial exposure, they are interpreted to have lithi¢ed in the marine environment because of evidence of primary marine cementation. The CQ lithofacies also contains francolite grains that are probably reworked from underlying Miocene Hawthorn beds, but it has not been correlated with any known lithologic unit. The interpretation of initial marine diagenesis within upper CQ beds is supported by evidence of primary marine cementation and microbially micritized skeletal grains (Fig. 3E). Relict textures of an isopachous, blocky to acicular cement (10 WmU40 Wm) typically line the substrate of pores ¢lled with drusy low-Mg calcite cement (Fig. 3B, E,F). These textures are interpreted to be the remains of high-Mg calcite cement, which is commonly precipitated during marine phreatic diagenesis. This interpretation is consistent with the absence of high-Mg calcite from outcropping beds. The quartz framework grains of upper CQ beds are situated in a micritic aragonite (?) matrix. The lower CQ units are interpreted as a lowenergy deposit based on their crystalline texture.
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Precursor sediments aggraded from mud-sized particles into low-Mg sparry calcite crystals with 87 Sr/86 Sr ratios that re£ect Quaternary (Table 1) seawater ratios. This sparry calcite resulted from subaerial exposure and subsequent meteoric diagenesis during a sea-level regression. The resulting 87 Sr/86 Sr ratio, therefore, probably re£ects the ratio (age) of the original sediment since excess strontium may not have been available in the meteoric environment to alter the 87 Sr/86 Sr ratio of the precursor mud. Calcite-rich hardbottom outcrops exhibit relatively high relief (up to 4 m) and are typically observed closer to shore than dolomitic hardbottoms (Fig. 1, Sites D, F, J, K). At two locations (Sites D, F), outcrops are underlain by the BGC lithofacies. 4.1.3. Blue-green clay (BGC) lithofacies The BGC lithofacies commonly crops out at the base of calcite-rich, scarped hardbottoms. The BGC lithofacies is primarily comprised of palygorskite with varying amounts of dolomite and quartz. This suggests that, based on mineralogy, a relationship exists between the underlying DQ lithofacies and the BGC lithofacies. Dolomite rhombs of the Arcadia Formation and Hawthorn Group are commonly draped with ¢brous Mgrich clays, such as palygorskite and sepiolite (Compton et al., 1994). In addition, on the westcentral Florida shelf, Brooks et al. (1999, 2003) have identi¢ed blue-green clay beds overlying bedrock and have suggested these clay beds may have formed as an alteration product of this underlying surface. CQ and DQ lithofacies have not been observed to crop out together on the inner shelf. However, if the blue-green clay represents the upper, weathered surface of the Arcadia Formation, dolomite (DQ lithofacies) probably underlies bedded calcite-rich outcrops. Alternatively, these palygorskite deposits may be detrital, originally forming in brackish-lagoonal environments and then being remobilized during Miocene sea-level £uctuations (Weaver and Beck, 1977). This interpretation ¢ts within the chronostratigraphic framework developed for the exposed hardbottoms, but does not explain its absence from the dolomite-rich hardbottoms.
301
4.2. Hardbottom formation Hardbottoms on the inner west-central Florida continental shelf are interpreted to be submarine hardground (non-deposition) surfaces, having formed under sediment-starved, low-energy conditions during sea-level highstands based on age data and sea-level studies (Haq et al., 1988) (Fig. 6). Based on seismic and core data at the mouth of Tampa Bay, Duncan et al. (2003) suggested the inner shelf experienced a time of relatively high sediment input and accumulation during the lowstands of the Late Miocene/Pliocene, potentially remobilizing sediment from interior Florida. This correlates with studies on peninsular Florida that indicate high sediment accumulation rates for the late Miocene/Pliocene Peace River Formation (Missimer, 1999; Guertin et al., 2000; Cunningham et al., 2003). High sediment accumulation would have prevented the formation of submarine hardgrounds, de¢ned as lithi¢ed surfaces of non-deposition. Sample ages fall into three categories, 15, 5, and 6 1 Ma. While this could be due to sample size, it may indicate that conditions were not conducive to the formation of hardground surfaces during much of the interval between the Miocene and the present. Dolomitic hardbottoms correlate with highstands at 15 and 5 Ma (Haq et al., 1988). Highstands of such magnitude allowed for topographically induced upwelling and an increased £ux of organic matter, promoting dolomitization (Compton et al., 1994). Based on the discussion in the previous section, Quaternary calcitic hardbottoms are also interpreted to have formed in the marine environment as a function of sediment starvation and minimal sediment movement (due to a low-energy setting) during a highstand. These outcrops have been diagenetically altered during a subsequent sea-level regression(s?). However, upper beds retained evidence of primary marine cementation, as well as evidence of deposition in a low-energy environment (¢ne grain size). The underlying sparry calcitic beds may also represent non-deposition surfaces, but extensive meteoric diagenesis has erased the original depositional texture of the sediments and any trace of primary marine cementation.
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Fig. 6. Chronostratigraphic column showing the timing of hardbottom formation with respect to sea level. Strontium isotope-derived ages for dolomite and phosphorite coincide with major highstands. The age of calcite hardbottoms indicates formation during the late Quaternary. Dolomitic units are a part of the Hawthorn Group. Older units are within the Arcadia Formation. The younger outcrops correlate with the age of the Peace River Formation in South Florida. Range of lithostratigraphic units based on Scott (1988) and Missimer (1999). Sea-level curve modi¢ed from Haq et al. (1988).
The resulting crystalline texture is inferred petrographically to have aggraded from ¢ne-sized particles, and, therefore, these beds are also interpreted as low-energy deposits. This indicates that the carbonate beds within outcrops, relatively unaltered calcite, (altered) sparry calcite, and dolomite may represent three di¡erent stages of hardbottom development. The dolomite formed during a time of high organic matter input onto the shelf (Compton et al., 1994), while the Quaternary outcrops lack this diagenetic component. This could be both a function of highstand magnitude, as a moderate or relatively short highstand may not create upwell-
ing conditions, and the young age of these outcrops. 4.3. Morphology One of the distinctive characteristics of the west-central Florida hardbottoms is the pervasive undercutting and scarp structure (Fig. 2A). Riggs et al. (1998) have attributed undercut hardbottoms in Onslow Bay to lithologic variability ^ a resistant lithology (to bio-erosion) overlying a less-resistant lithology. However, a visible di¡erence in lithology is not always present at the base of these outcrops. Only hardbottoms at sites F
Fig. 5. Idealized lithostratigraphic column showing the three distinct lithofacies and their mineralogy. Scale of photomicrographs is 0.25 mm. (A) Detrital grains exhibiting ¢rst-order birefringence are quartz in a micritic matrix. Blue epoxy resin shows porosity (HB-96-22; XN). (B) Micritic matrix has been replaced by low-Mg calcite spar. Individual crystals are up to 1 mm across and have replaced both mud and skeletal fragments, as well as encompassing multiple quartz grains (VC-96-14; XN). (C) Replacement sparry low-Mg calcite that range between V0.1 and V0.05 mm (HB-96-33; XN). (D) Unconsolidated BGC from the base of a sediment vibracore. Core barrel is 7.6 cm (3 in.) in diameter. (E,F) Hawthorn Group dolomite with high amount of quartz grains (VC-96-13; XN) and low amount of quartz grains (HB-96-18; PPL), respectively.
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and D are underlain by blue-green clay. Since all observed outcrops are indeed undercut, either subtle changes in lithology or as yet unidenti¢ed blue-green clay layers exist at the base. A ¢nal alternative is that lithology may not be the dominant factor controlling the undercut morphology of scarped hardbottoms. Riggs et al. (1998) have suggested that fracture patterns on the North Carolina continental shelf may play a role in the orientation of scarped hardbottoms. In general, the outcrop features of west-central Florida are roughly shore-parallel (330‡^0‡) although variable over short distances (Fig. 2B). Vernon (1951) identi¢ed fracture patterns that trend northwest^southeast in west-central Florida based on aerial photographs. It has been suggested that these fracture patterns control the modern coastal morphology of the marsh coast, north of the Tampa Bay area, due to preferential dissolution of limestone (Hutton et al., 1984; Hine et al., 1988). It is possible that the outcrop systems o¡ west-central Florida are another manifestation of this structural control. 4.4. Implications for modern sedimentary development Today, £uvial input accounts for little, if any, of sediment on the shelf (Brooks et al., 2003). McNeil (1949) noted several subaerial beach ridges, relicts of Pleistocene sea-level transgressions, that may be a source of siliciclastic sediments during highstands. Riggs and O’Connor (1974) and Winston et al. (1968) indicated that there is no obvious, outside source of siliciclastic and phosphatic sediments and suggested nearshore hardbottoms to be a source for these sediments. The hardbottoms of west-central Florida are undergoing bio-erosion. Testing the hypothesis that sediments are winnowing from outcropping hardbottom strata, Berman (1998) statistically compared the size and shape of sur¢cial phosphatic sands collected adjacent to scarped hardbottoms with those incorporated within the hardbottoms and found them to be of the same statistical population. By analyzing over 500 bottom samples, Berman (1998) also determined the
quantity of phosphorite grains present in the sediment veneer to be inversely related to the thickness of the veneer, indicating a relationship between phosphorite in the surface sediments and the underlying hardbottoms. Direct observations by SCUBA divers have shown that the percentage of phosphorite grains increases dramatically with proximity to hardbottom, and have con¢rmed that large quantities of phosphorite grains are present in areas of thin sediment cover. Bio-erosion of outcropping Upper Cenozoic strata in Onslow Bay is facilitating the contribution of signi¢cant volumes of new sediment to the continental shelf at rates of 5.5, 0.4, and 0.03 kg m32 yr31 , depending on morphology and lithology (Riggs et al., 1998). Using the median bio-erosion rate of Riggs et al. (1998) as the best analog for west-central Florida morphology and lithology, and the estimate that hardbottoms contain a mean percentage of 20% quartz grains and 5% phosphorite grains (Table 1), a mass of 0.08 and 0.01 kg m32 yr31 of siliciclastic and phosphatic sediment is potentially released by scarped hardbottoms. Onslow Bay hardbottoms produce sediment rapidly enough to bury the contributing outcrop; however, storms modify the sediment distribution and remove large volumes from the shelf, resulting in minimal sediment accumulation (Riggs et al., 1998). The west-central Florida hardbottoms have minimal sediment cover, and there is little evidence that large-scale transport of sediments is occurring on the shelf (Brooks et al., 2003; Edwards et al., 2003; Harrison et al., 2003); therefore, qualitatively, it appears that less sediment is produced via hardbottom bio-erosion than in Onslow Bay.
5. Conclusions Two types of hardbottoms (dolomitic and calcitic) have been identi¢ed on the inner west-central Florida continental shelf. They trend roughly shore-parallel, exhibit varying relief (up to 4 m), and are rich in siliciclastic and phosphatic grains. Each type provides a substrate for diverse £oral and faunal assemblages that encrust and bore the
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outcrop, especially on the lee (shoreward) side of the feature, possibly contributing to the shoreward-facing scarp. This bio-erosion may also contribute to the undercutting which results in the failure of overhanging blocks that periodically fail, producing a rubble ramp/¢eld. This serves as a mechanism for releasing sediment grains onto the inner shelf at a potential rate of 0.04 kg m32 yr31 of siliciclastic sediment. Relatively high-relief (up to 4 m) Quaternary calcitic hardbottom outcrops that are occasionally underlain by a layer of blue-green clay have not been correlated to any formational unit. Low-relief ( 6 2 m) dolomitic hardbottoms are extensions of the underlying, Miocene Hawthorn Group. Hardbottoms likely formed as non-deposition hardground surfaces during periods of sediment starvation and low-energy conditions associated with high sea level. The di¡erence in lithology is a function of time and diagenesis ; however, the lack of a high organic matter content in Quaternary sediments may slow or prevent the Quaternary hardbottoms from altering to dolomite. The prevalence of undercutting (scarped hardbottoms) may be a function of lithology, although this is still uncertain. The BGC lithofacies, while present beneath Quaternary outcrops, has not been identi¢ed beneath scarped dolomite-rich outcrops.
Acknowledgements This project was funded by the USGS WestCentral Florida Coastal Studies Project and the Howard Hughes Medical Institute. Eckerd College and the Alyesworth Foundation for the Advancement of Marine Science provided ¢nancial support for S.P.O. We would like to thank S.R. Riggs and W.W. Schroeder for their thoughtful reviews, as well as Bob Halley and Bret Jarrett for sharing their extensive knowledge of carbonate diagenesis. Assistance in the ¢eld was provided by David Bennett, Beau Suthard, Greg Berman, and Alex Moomaw, and the multibeam image was provided by David Naar. For assisting with just about everything, Meghan Pitchford and Boudewijn Remick deserve our deepest gratitude.
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