On the Sarasota Arch, the depth to the top of a 5.8-5.9 km/s layer is 3-4 km below sea ... on the Florida platform in the southeastern Gulf of Mexico (Figure 1). ...... Buffer, R. T. (in press), "Seismic stratigraphy and Geologic History of the Deep ...
CRUSTAL
STRUCTURE
FLORIDA GULF
OF
OF THE
PLATFORM,
MEXICO:
SEISMOGRAPH
AN
SOUTH
EASTERN
OCEAN-BOTTOM
REFRACTION
STUDY
J O S E P H O. E B E N I R O , W I L L I A M P. O ' B R I E N , Jr., and F. J E A N N E S H A U B Institute [or Geophysics, The University of Texas at Austin, 4920 North I.H. 35, Austin, T X 78751, U.S.A.
(Accepted 18 July, 1986)
Abstract. Five seismic refraction lines, 70-90 km long, were shot in the South Florida Platform region of the Gulf of Mexico using digital ocean-bottom seismographs. Apparent velocities and depths were calculated from the refracted arrivals using a fiat-layer model for the region. The two dominant refractors have apparent compressional-wave velocity ranges of 5.6 to 5.9 km s -~ and 6.2 to 6.7 km s-t. On the Sarasota Arch, the depth to the top of a 5.8-5.9 km/s layer is 3-4 km below sea level. This depth corresponds to the depth to the crystalline basement. The basement dips to the north and to the south from the arch, with velocity of the upper crust increasing from 5.8-5.9 km s ~ to a maximum of 6.7 km s ~ at a depth of 6.3 km. Under the continental slope, the crust has presumably been thinned and extended. The deepest refractor has an apparent velocity of about 7.5 km s t at a depth of 25 km. The thickness of the crustal section and the absence of any mantle arrivals in these long refraction profiles on the platform suggest that thick continental crust underlies the South Florida Platform. A north-south cross-section through the platform suggests the presence of two structural highs separated by a portion of the South Florida Basin, which contains at least 5 km of sediment.
1. Introduction A r e f r a c t i o n e x p e r i m e n t using digital o c e a n - b o t t o m s e i s m o g r a p h s ( O B S ) was c o n d u c t e d in 1982 by the U n i v e r s i t y of T e x a s Institute for G e o p h y s i c s ( U T I G ) on the F l o r i d a p l a t f o r m in the s o u t h e a s t e r n G u l f of M e x i c o ( F i g u r e 1). T h e study a r e a is l o c a t e d on the s o u t h e r n shelf and u p p e r slope of the p l a t f o r m , a m a j o r c a r b o n a t e p r o v i n c e that f o r m s the e a s t e r n b o u n d a r y of the G u l f basin. T h i s r e g i o n has b e e n difficult to study. P r o v e n g e o l o g i c a l e x p l o r a t i o n t e c h n i q u e s such as s o n o b u o y r e f r a c t i o n and m u l t i c h a n n e l r e f l e c t i o n s u r v e y s c o n d u c t e d in the r e g i o n p r i o r to this study h a v e g i v e n p o o r results, d u e m a i n l y to p o o r e n e r g y p e n e t r a t i o n and s t r o n g b a c k - s c a t t e r i n g c o m p o u n d e d by t r a p p i n g of l o w f r e q u e n c y e n e r g y in shallow strata ( T a t h a m and G o o l s b e e , 1984). T h e c h a r a c t e r of b a s e m e n t u n d e r l y i n g the a r e a had not b e e n well defined. A n t o i n e and E w i n g (1963) s h o t s e v e r a l r e f r a c t i o n dip lines across the p l a t f o r m , but the crustal i n t e r p r e t a t i o n of t h e s e lines was n o t c o n c l u s i v e b e c a u s e t h e y r e c o v e r e d v e r y f e w d a t a f r o m d e p t h . T h e y did identify a r e g i o n a l 5.8 k m s -~ r e f r a c t o r , at a d e p t h r a n g e of 2 . 6 - 3 . 6 k m but this v e l o c i t y c o u l d be a s s o c i a t e d with e i t h e r crystalline b a s e m e n t or c a r b o n a t e sections. K r i v o y and Pyle (1972) Marine Geophysical Researches 8 (1986) 363-382. 9 1986 by D. Reidel Publishing Company
364
280
J O S E P H O, E B E N I R O E T
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to 2~o
:::::::::::::::::::::::::::::::::::::::::::::::::::::::::,:,
EMBA'~' ENT
!
3.
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oBs
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2, ~
-
"X~o 'LOBS 3
Z
~
-
~i
-
J
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,r-'---~_ t ~
Oo
Jorda ~ Knoll ~) 23 ~ -85 ~
~ -84 ~
"~..~ ~ ~ . . ' ~ . -83 ~ -82 ~
-81Q
Fig. 1. Map of the southeastern Gulf of Mexico showing the location of the five refraction lines. Large dots indicate the locations of the ocean bottom seismographs used in the experiment. concluded from their interpretation of a B o u g u e r gravity anomaly map of the west Florida margin that the crust underlying the T a m p a E m b a y m e n t is intermediate between that of c o n t i n e n t s and of oceans. Based on their analysis of seismic refraction, magnetic, gravity and heat-flow data, Martin and Case (1975) suggested that this area has a c o n t i n e n t a l foundation. Klitgord et al. (1984)
CRUSTAL
STRUCTURE
OF
SOUTH
FLORIDA
PLATFORM
365
considered the platform as part of a transform plate boundary underlain in large part by oceanic or thinned rift-stage crust with mantle rocks accordingly at a relatively shallow depth. They based this interpretation on plate reconstructions inlegrated with interpretations of magnetic, gravity, seismic and deep drill-hole data. Crystalline basement is probably overlain by restricted-circulation, shallowmarine carbonates and evaporates of Jurassic(?) through Early Cretaceous age (Bryant et al., 1969; Mitchum, 1978; Freeman-Lynde, 1983). Extrapolation from peninsular wells indicates that the younger section underlying the shelf is likely to be a continuation of these non-clastic lithologies (Maher and Applin, 1968). The Upper Cretaceous-Holocene section under the upper slope is composed of open-circulation, deep-water carbonate and chalk deposits (Mitchum, 1978; Freeman-Lynde, 1983; Holmes, 1985). The data presented in this paper from five large-offset refraction profiles that crossed the shelf and slope of the southern platform led us to conclude that the Florida platform is part of the continental margin of the Gulf of Mexico and is not underlain by oceanic crust.
2. Equipment The digital OBS used in these experiments (Figure 2; Latham et al., 1978; Steinmetz et al., 1979; Nakamura, 1983) was designed and built at U T I G . This integrated instrument package consists of a triaxial geophone system with recording and control electronics housed in a glass sphere, 43 cm in diameter, which fits snugly in a modeled plastic cap. The sphere with its contents is secured firmly to a heavy metal frame footing with three stiff elastic straps linked by stainless steel wire. The frame is 1.2 m square and has many spikes which penetrate the ocean floor on impact with sediments to improve seismic coupling. The sensor system consists of one vertical and two horizontal geophones. A special unit which contained three vertical geophones was used as OBS 1 on the first four lines (1-4). The natural frequency of these special geophones were 2.0, 4.5, and 10.0 Hz respectively. The frequency of the vertical-component geophones in all the other units was 4.5 Hz, and the frequency of all horizontal component geophones was 10.0 Hz. Each OBS contained electronic clocks and three microprocessors programmed to activate the instrument at specific time for each shot to detect, digitize, multiplex and record the shot on a 4-track cartridge tape. Reference timing for each OBS was provided by the internal clock, which was calibrated against WWV immediately before each deployment and immediately after each recovery. The sampling rate of the instrument for this experiment was 136 samples/sec and the dynamic range was about 96 dB. All OBS units had a compass mounted externally a few centimeters above the glass sphere. The compass locks into position some hours after deployment on the
366
JOSEPH
O. E B E N I R O
ET
AL.
Flag
Radio beacon
Compass~/ Release mechanism
Buoyant glass sphere
Metal frame
Fig. 2. The University of Texas Institute for Geophysics OBS used in this experiment mounted in its spiked, square anchoring frame.
ocean floor, allowing the orientation of the two horizontal geophones to be determined. At deployment, the OBS, m o u n t e d on the frame, was released from the sea surface and allowed to free-fall to the seafloor. At the end of data acquisition, an electric c u r r e n t causes the electrochemical dissolution of the stainless steel wire holding the i n s t r u m e n t sphere onto the metal frame. This allows the sphere with its contents to float to the surface for recovery. Each unit is equipped with strobe lights and two radio beacons that would activate when the unit clears the water. Two small bright flags were also attached to the b e a c o n a n t e n n a s to aid further in locating the OBS at the surface. T h e response of the OBS to seafloor motion depends on at least four separate factors: the integrity of the coupling between the OBS frame and the seafloor, the structure and rigidity of the OBS frame itself, the geophone response and the
CRUSTAL
t
m 9"o
STRUCTURE
OF
SOUTH
i
l
i
~ i i
]
]
]
i iiiJ]
FLORIDA
rI
367
PLATFORM
i
i
i
i
llll
o
O9 Z -lo
209
iii .Jer,~-20 / ~_~
,/j,/~j
-30
O ~40
L
0.2
i
i
i
i i
1
]
10
1OO
FREQUENCY (Hz) Fig. 3. Nominal relative instrumental response to ground motion for the geophones and the alias filter for the UTIG OBS used in the experiment. Note the broad passband of the response which contains most of the expected signals.
electronic alias filter in the preamplifiers. The coupling of the OBS frame to the ocean floor remains a classic unresolved problem to marine seismologists (Sutton et al., 1981). The low-pass alias filter had a corner frequency of about 3 l Hz and a rolloff of 24 dB/octave at frequencies above 31 Hz. Although we did not carry out any resonance tests on the frames used in this experiment, we presume that the frames have resonance frequencies greater than 24 Hz. The resonance frequency of a prototype frame built less rigidly than the frames we used was 24 Hz (Steinmetz et al., 1979). The nominal OBS response to ground motion of these units (Figure 3) shows a broad passband which contains most of the signals of interest in this experiment. The sound sources for each line were 61 explosive shots (DuPont T o v e x Extra) ranging in size from 13.6 to 81.6 kg, which were detonated at five-minute intervals and were distributed in such a way that the small size charges were in the middle of the profile line while the largest charges were detonated at both ends of the lines.
3. Field Techniques This experiment was carried out on the R/V Ida Green. T-he procedure followed was to deploy three OBS's at predetermined locations along a straight line. We retraced this line at 10 knots while dropping explosive charges at five-minute intervals (shot spacing of about 1.5 km) and finally, we retraced the line for the third time to recover the instruments. Bad weather was encountered for lines 3 and 4, with the result that the shot coverage for line 3 extended only to about
368
JOSEPH O. EBENIRO ET AL.
Table I End coordinates of the seismic lines Line No.
Shot '1
1 2 3 4 5
25~ 26~ 25~ 24~ 24~
Shot 61 84~ 83~ 83~ 83~ 83~
Length (km)
26~ 25~ 25~ 25~ 24~
84~ 84~ 83~ 82~ 83~
84 93 69 83 8(I
Table II OBS Locations Line No.
OBS No.
Latitude
Longitude
Depth (m)
1
1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
26~ 26~ 25~ 25~ 26~ 26~ 26~ 25~ 25~ 25~ 25~ 24~ 24~ 24~ 23~
84~ 84~ 84~ 84~ 83~ 83~ 83~ 83~ 83~ 82~ 83~ 1.08'W 83~ 83~ 83~ 83~
210 216 214 144 75 55 99 77 70 44 57 68 62 558 916
2
3
4
5
6 9 k m . O u r p r i m a r y n a v i a t i o n w a s L o r a n - C e x c e p t w h e n we lost t h e s i g n a l a n d w e r e f o r c e d to rely o n s a t e l l i t e d a t a . I n all, five lines, w i t h e n d c o o r d i n a t e s s h o w n in T a b l e 1, w e r e s h o t d u r i n g t h e e x p e r i m e n t . O f t h e f i f t e e n O B S ' s d e p l o y e d , t h i r t e e n r e t u r n e d d a t a ( T a b l e II). The middle OBS's on lines 1 and 3 malfunctioned and recorded no data. The shot times were determined by comparing the recorded signals from the towed hydrophone with the recorded time record from an electronic master clock, t a k i n g i n t o a c c o u n t t h e e l a p s e d t i m e b e t w e e n t h e a c t u a l s h o t a n d t h e d e t e c t i o n of t h e s h o t b y t h e h y d r o p h o n e . T h i s m a s t e r c l o c k w a s also u s e d to d e t e r m i n e t h e d r i f t of e a c h O B S c l o c k d u r i n g d e p l o y m e n t .
4. Data
Processing
and Reduction
T h e d i g i t a l d a t a r e c o r d e d o n e a c h 4 - t r a c k c a r t r i d g e t a p e w e r e t r a n s f e r r e d to a 9 - t r a c k t a p e in s t a n d a r d S E G - Y
trace-sequential
f o r m a t ( B a r r y et al., 1 9 8 0 )
CRUSTAL
STRUCTURE
OF SOUTH
FLORIDA
PLATFORM
369
Fig. 4. Composite seismic section for Line 4. The data traces have been gained and filtered. The interpreted velocity values are derived from fitting the arrivals with least-square straight lines.
370
JOSEPH
O. E B E N I R O
ET
AL
D I S T A N C E (KM) Sarasota/Pinellas County
Tampa
Arch
OBS 3 9 10 '
S
/ ~~ . 6 s
^-z.(4
20
30
~
I
40
50
I
' ic 1 5 2 ~
= l.~2_Upoer Ce .... . . . . . . . .
60
OBS 1
i
~.~_
5.54 --,~----2 .8 ~ _ P, L o w e r C r y , 5 58 6.02 - - ~ ~ ~ ~ L._~'aCeous " I' ~8 T o o of B a s e m e n f 6.30
5 10
90
80
i9
Embayment ),
9
5
Intra-crustal Arrivals
6.60
oN
10
6.88
15
15 Line 1 (a)
VE=2-0 2O
OBS 1
SWo
9
I
~-J~8_ 5.22
,r
I
I
5
I
j~8S" - - -
2.4Sh~llo w
9 5.95
'~ "T" I-Q. UJ a
2O OBS 3
OBS 2 9
2.5z5,10
I 492
I
~-----1 . . . . 2'.5
I
I
--H~i~"-~ - . . . . .j~_~.-,.-_ Velocity 5.8 1 I 5.83 Horizon T o p of B a s e m e n t
_
9
-f'-" _:~:~1_ . 0
"~--'L~2.5
4.62 t
5.56 ~-.-~------.-O 5.88
lO
6.48
15
9
Intr3-crlJstal Arrivals
9
6,79
15
703
Line
VE=2.5
2O Sarasota/Pinellas
OBS 3 A
f
2
4 ~ 3 6 ~ " 9
5
I
I
I
3 5 - -_-/E~_-_-_- - - :
I
I
I
~A
- _ _ 2.a5__ - * ~ _ _ ~
Upper / TopPLowe r ~,enozotc Creiaceous 5.82 f Post-rill Sag Basin Horizon ~ ~ ~ 9 6.33
10
5.54~ 5.88
8
Arch
o NNW 5
10
t . . . . . .
Arrival from Pre-rifl Section
6.79
Q
15
15 VE=2.5
20
County
OBS I
~
2
(b)
20
o
m 'o --N -I-
5
10
SSE
NE
Line 3 (c)
2O
CRUSTAL
STRUCTURE
OF
SOUTH
FLORIDA
DISTANCE OBS 3
W
0
9
I
463566 /
5
(KM)
OBS 2 J
_ ~ 1H -/,v~____~-..,C-~ -
I
2.05
p
~ -
Upper Cenozoic
A ~ - 2 _ ~ '
~ 3 4.4~
OBS ,
,
05
-
~
--------6'51 _
l -- ~ ~
636~'
~//
TOp
of
-- -
~
5 ~
10
9
Basement
15
6.39
20
20
A
Intra
crustal
25
V
(3
Arrival
7.47
Line 4 (d)
VE=2-5
OBS 3
OBS 2
oN - 5.91
MCU
9 6.26 -- _ -- -- --- tL-6.67
10
t
6.20
9
--
5
9 6.87
Interpreted Top of Basement
10 15
15 20
6.65
25
9
20
Intra-crustal Arrivals
7.20
VE=2"5
30
"13 "-I "1"
OBS
0 5
25
rn
3O
30
S
oE
Arrival
I/
"1" I-13. ILl 13
,A
Top, Lower Cretaceous
5.95
/ l_ -
10 15
,
..- ...................--~ 2.14r- ? 3.85
_. _ --)-g~
9 -- . . . . . ~ Post-rift Sag Basin Horizon
~
371
PLATFORM
Line 5 (e)
25 30
Fig. 5. Composite velocity-depth profiles for the lines {Lines 1-5) calculated using a model for a fiat-layered homogeneous medium. Large dots show the calculated points of critical refraction, and the horizontal bars extending from the dots represent the refracting interface with the calculated apparent velocity (kin s-~) shown below the bar. The dashed lines represent the correlated interpretations from associated multichannel reflection lines in the area.
372
JOSEPH
O. E B E N I R O
ET
AL.
incorporating corrections for shot time, clock drift and the shot-to-hydrophone travel times so that the first sample on each trace corresponded to the actual shot instant. The timing correction for the distance between the shot and the hydrophone was estimated using their horizontal separation and the depth of the shot determined from the bubble pulse data using the Rayleigh-Willis equation (Kramer et al., 1968). We used 1.52 km s -~ as the velocity of sound in water for these calculations. The processed data from the vertical component geophone (4,5 Hz natural frequency) from each line were analyzed by preparing seismic record sections of all 61 shots. The data were filtered using a minimum-phase 3-pole Butterworth filter with a 2.5-8 Hz passband to reduce the noise and enhance the ability to pick first arrivals. We also scaled all trace amplitudes for shot-OBS distance and shot weight effects using a trace-amplitude scale factor (Orcutt et al., 1976) K
(R/R.)(W./W)
"'5
(1)
where R, = 10 km, Wo= 601b, R = shot-OBS distance and W = shot weight. Figure 4 represents an example of the composite record section obtained in this experiment9 Time picks of primary and secondary arrivals on these record sections were corrected for topography using the methods of Withmarsh (1975) and Purdy (1982), and the apparent velocity of the arrivals computed using least-square fits to straight-line segments of the traveltime picks. The reversibility criteria (Ewing et al., 1939) for the reciprocal and intercept times generally were not satisfied for these data, a condition we ascribe either to the presence of lateral inhomogeneities along the lines or to velocity anomalies at either the shot or OBS locations (Warren et al,, 1966). Therefore, assuming a model of laterallyhomogeneous horizontal layers, "we computed the layer thicknesses separately for each OBS location. We believe that the apparent velocities and depths inherent in using this model differ from the true values by less than 1t)%, since the geological setting is relatively flat in most areas of the platform. Once exception is line 5, where significant changes from the apparent values arise as a result of the shelf/slope break. The unreversed velocity-depth data for each line are plotted in Figure 5.
5. Data Interpretation Tile wide shot spacing and consequent sparseness of the data made first-arrival observations of shallow layers impossible. For these layers, the velocities and depths were either obtained from previous refraction studies with closer shot spacing (Antoine and Ewing, 1963) or inferred from reflection data (Shaub, 1984). The uncertainty (one standard deviation) in the calculated apparent velocities due to scatter of the travel-time data used for the least-square fits was generally on the order of 0.05 km s -~, unless otherwise noted.
C R U S T A L S T R U C T U R E OF S O U T H F L O R I D A PLATFORM
5.1. L I N E
373
I
This line was shot on the basinward flank of a Paleozoic c o n t i n e n t a l crustal b l o c k ( K l i t g o r d et al., 1984) called the Sarasota A r c h (Pinellas C o u n t y A r c h , Buflter et al., 1984) in water depths of a b o u t 200 m. T h e shallowest layers discernible as first arrivals at O B S 1 and O B S 3 had m e a s u r e d P-wave velocities of 3.0 km s -~ and 2.7 km s -~, respectively (Figure 5a). T h e top layer, of velocity 1.65 km s -~, was inferred from nearby reflection data. T h e section with velocities ranging up to 5 . 6 k m s -~ t h i c k e n e d to the north towards the T a m p a E m b a y m e n t . T h e absence of the 4.85 km/s layer at O B S 3 s u g g e s t e d that this layer thinned towards the Sarasota A r c h . T h e top of the o b s e r v e d 5.6 km s -~ layer d i p p e d towards the T a m p a E m b a y m e n t from 2.3 km d e p t h at O B S 3 to 3.2 km d e p t h at O B S 1. T h e 6.1) km s -~ and 6.3 km s -~ arrivals from r e s p e c t i v e depths of 3.7 km at O B S 3 and 5.5 km at O B S 1 were i n t e r p r e t e d as c o m i n g from the top of crystalline basement. T h e r e f o r e the u p p e r crustal b o u n d a r y on the seaward flank of the Sarasota A r c h has an a p p a r e n t northerly dip towards the T a m p a E m b a y m e n t . T h e same structural trend was indicated by Burlier et al. (1984). An inferred intracrustal layer of velocity 6 . 6 - 6 . 9 km s -~ was o b s e r v e d at both O B S positions at depths of 10-1 l km. 5.2.
LINE
2
This line was shot along the crest of the Sarasota A r c h in water depths ranging from 55 m at O B S 3 to 144 m at O B S 1 (Figure 5b). We assumed the existence of shallow layers with velocities of 1.8 km/s and 2.4 km s -1 at O B S 1 (Profile 4 of A n t o i n e and Ewing, 1963). A t O B S 2 and O B S 3, the assumed layers had r e s p e c t i v e velocities of 1.8 k m s 1 and 2.5 k m s -I ( A n t o i n e and Ewing, 1963; s e c o n d arrivals from Profile 3). A notable feature was the shallow h i g h - v e l o c i t y layer. All three O B S units r e c o r d e d near-offset arrivals from this l a n d w a r d - d i p p i n g layer, whose velocity r a n g e d from 4.6 • 1.7 km s -~ at O B S 3 to 5.2 km s -~ at O B S 1. T h e 5.8-5.9 km s - ' layer was also o b s e r v e d on all the receivers, and the top of this layer had a slight landward dip from about 3.0 to 4.0 km depth. W e interpreted this layer as being at or near the top of crystalline basement, since the velocity was a p p r o p r i a t e for continental crust and the depth in a g r e e m e n t with published b a s e m e n t structure (Buflter et al., 1984). A crustal layer with velocity of 6 . 5 - 7 . 0 km s -~ at an a p p a r e n t d e p t h of 14-15 km was o b s e r v e d by all three O B S units. 5.3. LINE 3 Line 3 (Figure 5c) was shot on the southern flank of the Sarasota A r c h in water depths varying from 99 m at O B S 1 to 7 0 m at O B S 3. A l t h o u g h this line was p l a n n e d to be 90 km long, high seas from an u n f a v o r a b l e direction affected the ship speed so that the actual length of the line was 69 km, and the last shot was positioned 17 km south-southeast of O B S 1 (Figure 1). Because of this large
374
JOSEPM
O. E B E N I R O
E'r AL.
m i n i m u m offset for O B S 1, the shallowest layer r e c o r d e d as a first arrival by O B S 1 had a velocity of 5.5 km s - l . H o w e v e r , by studying later arrivals, we found layers with velocities 2.35, 3.45, and 4.9 km s -~ a b o v e the 5.5 km s -~ layer at the O B S 1 location. B e n e a t h O B S 3, the shallowest layer o b s e r v a b l e from first arrivals had a velocity of 4.4 k m s - l , and from the later arrivals we found 2.35 and 4.9 km s -~ layers. W e i n t e r p r e t e d the 5.9 km s -1 arrivals at O B S 1 from 4.2 km depth and the 5.8 km s -1 arrival at O B S 3 from 4.6 km depth as a seismic horizon l o c a t e d in the lower part of a postrift s e d i m e n t a r y basin ( K l i t g o r d et al., 1984) near the top of basement. T h e 6.3 km s -I arrivals from O B S 3 were i n t e r p r e t e d as i n t r a - b a s e m e n t arrivals at 8.9 km depth, as were the arrivals with a velocity of 6.8 km s -I from a depth of 12.6 km d e t e c t e d by O B S 1. T h u s the b a s e m e n t lies b e t w e e n 4.6 km and 8.9 km. 5.4. LINE 4 Line 4 (Figure 5d) ran a p p r o x i m a t e l y east-west, with water depths c h a n g i n g from 44 m at the east end (OBS 1) to 68 m at the west end (OBS 3). This line lay south of and almost parallel to profiles 1-4 of A n t o i n e and Ewing (1963) in the area usually referred to as the South F l o r i d a Basin. T h e shallowest layer r e c o r d e d on this line had a p p a r e n t velocities varying from 3.85 km s - l at O B S 1 to 4.8 km s -I at O B S 3. T h e lower-velocity layers (2.1, 2,05 and 2.2 km s - l ) were d e d u c e d from nearby reflection data (Shaub, 1984). Arrivals in the velocity range of 5.9 to 5.95 km s - l were r e c o r d e d by O B S 1 and O B S 2. D e p t h s to the top of this layer increased from 3 km near the western end of the line (OBS 3) to a m a x i m u m of 4 km near the c e n t e r of the line (OBS 2) and b e c a m e shallow again with a d e p t h of 2.3 km at the nearshore end (OBS 1). W e i n t e r p r e t e d this surface as the top of a s e d i m e n t a r y feature within the post-rift basin. B e n e a t h this layer, at a d e p t h of about 8 - 9 km in the east and 14 km in the west, was the 6.4-6.5 km s -1 layer which we i n t e r p r e t e d as the b l o c k - f a u l t e d top of basement. T h e top of a p r e s u m e d crustal layer with a velocity of 7,5 km s - l at a d e p t h of 24.5 km was r e c o r d e d by O B S 3 alone. 5.5. LINE 5 Line 5 (Figure 5e) was shot across the shelf b r e a k in water ranging in depth from 62 m at O B S 1 to 916 m at O B S 3. Line length was about 82 km. T h e middle O B S was l o c a t e d about 15 km south of the shelf b r e a k at a d e p t h of 558 m. A r r i v a l s in the velocity range of 4.8 km s -1 at O B S 3 to 5.3 km s -1 at O B S 1 defined the top of a s e a w a r d - d i p p i n g layer extending from the shelf to the u p p e r continental slope under the northern Straits of Florida. U n d e r l y i n g this layer, with an a p p a r e n t velocity range of 6.2-6.7 k m s -1, was the i n t e r p r e t e d b a s e m e n t , which d i p p e d southward from a d e p t h of a b o u t 5.3 km under O B S 1 to a b o u t 6.3 km u n d e r O B S 3. A d e e p crustal layer with velocity of 7.2 km s -I was o b s e r v e d at a d e p t h of 25 k m at the O B S 3 site only. T h e d e p t h of 20 km to the top of a layer with an a p p a r e n t velocity of 6.65 km s -~ o b s e r v e d at the O B S 1 location is questionable, since the arrivals from this layer
CRUSTAl_
STRUCTURE
OF SOUTH
FLORIDA
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375
traversed three different topographic regions (the deep slope, the shelf break and the shallow shelf) whose unknown but potentially complex crustal structure would have invalidated the simple layer-solution approach used in this analysis. 5.6. ALL LINES The observed apparent velocities and suggested geologic ages of the various units identified above are given in Table III. A brief summary follows. The Mesozoic-Holocene sediments that underlie the shelf are, as noted previously, expected to be a section similar to that encountered in peninsular wells, i.e., redbeds overlain by shallow marine carbonates and evaporite facies (Maher and Applin, 1968). Under the upper slope, the sediments overlying basement through the Lower Cretaceous most probably comprise a similar section; these sediments constructed the platform margin known as the Florida Escarpment. During Late Cretaceous-Early Cenzoic time, however, the platform margin retreated from the escarpment to a more landward location. Accordingly, the depositional section underlying the upper slope indicates a transition upward in section from platform margin and interior facies to deep-marine limestones and chalk (Freeman-Lynde, 1983). The Upper Cenozoic slope section is a reversal of this trend; on the southern platform upper slope, this section represents an open-marine progradational regime consisting of hemipelagic carbonate sedimentation (Holmes, 1985). In general, then, the basal-through-Lower Cretaceous sedimentary rocks, whether sampled on the shelf or slope, should be an identical suite of lithofacies. The P-wave velocity range for this interpreted geologic-age section are also very similar, shelf versus slope: 4.9-6.0 km s -I (shelf) and 5.1-6.3 km s -~ (slope). The Upper Cretaceous-Cenozoic rocks, on the other hand, seem to be differentiable, shelf versus slope, by lithology, depositional regime and also by refraction velocity. Specifically, the velocities are consistently higher on the shelf. This relationship is not the result of uncorrected arrivals from downslope stratal dips; such a dipping section would actually reverse this velocity relationship. Rather, we attribute this difference between rocks from different depositional regimes to the variation in
Table III Correlation of apparent velocityranges with geologic age dates Geologic ages
Velocity range (km s-~)
Late Cenozoic
1.6-1.8 (upper 100 m sub-bottom) 2.0-2.8 (shelf) 1.7-2.4 (otherwise) 3.7-5.2 (shelf) (4.4--4.9 typical) 2.4-3.2 (slope) 4.9-6.0 (shelf) (5.5 typical) 5.1-6.3 (slope)
Early Cenozoic-Late Creataceous Cretaceous
376
JOSEPH
O.
EBENIRO
ET
AL.
physical characteristics such as cementation, diagenetic properties and depositional mechanisms. Basement velocities vary with interpreted depth to basement on Line 2, a strike line along the crest of the Sarasota continental crustal block (characterized by a broad gravity minimum), sampled basement velocities were 5.8-6.0 km s -t. On Line l, the basinward flank of the arch, we interpret the 6.0-6.3 km s -j layers as the top of basement and note that the basement surface and the internallythickening sediment section dip seaward and northward into the Tampa Embayment. On Lines 3, 4 and 5, we observe basement velocities which generally increase from about 5.9 km s-' at the southern flank of the Sarasota Arch to about 6.9 km s -~ towards the upper slope at Line 5. Presumed intra-basement arrivals were interpreted on every line with apparent velocities ranging from 6.4 to 7.5 km s -1 and with apparent depths ranging from 15 to 25 km.
6. Regional Interpretation The interpreted data from the five lines have been combined to produce a north-south composite cross-section of the South Florida Platform (Figure 6). Figure 7 shows the location of the interpreted OBS refraction lines superimposed upon Bouguer gravity anomalies and the line of section. On the basis of the refraction velocities and depths, structural analysis of associated multichannel lines (Shaub, 1984), the Bouger gravity features (Martin and Case, 1975) and available peninsular geological information (Maher and Applin, 1968), we suggest that the
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Fig. 7. Bouguer gravity anomaly map of the southeastern Gulf of Mexico modified from Martin and Case (1975). The gravity contours are given in milligals. The locations of the five large-offset lines (heavy) used in this study and the north-south cross-section (light) are shown. The locations of two deep basin refraction lines (Line 12; Ibrahim et al., 1981 and Line 6; Ebeniro et al., 1984) are also indicated in heavy dashed lines. The locations of the structural basement highs (Sarasota Arch and Sheffield Arch) and the downthrown block interpreted from the refraction, MCS, well and gravity data are also shown.
S o u t h F l o r i d a P l a t f o r m is c o n t r o l l e d by: (1) t h e S a r a s o t a A r c h , a s h a l l o w c r u s t a l f e a t u r e c o i n c i d e n t w i t h a b r o a d 0 - r e g a l g r a v i t y m i n i m u m ; (2) a s e c o n d d o w n t h r o w n b l o c k to t h e s o u t h e a s t of t h e S a r a s o t a A r c h , c h a r a c t e r i z e d b y a 1 0 - m g a l g r a v i t y contour and having a 20-mgal gravity contour with the same gravity anomaly w a v e l e n g t h as t h e S a r a s o t a A r c h . S e v e r a l of t h e O B S lines a r e s i t u a t e d p a r t i a l l y o n
378
JOSEPH
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ET
AL.
the crest or on the seaward flank of this double-block system and give structural information about these features and their basinward margins. The composite profile begins on the northwestern flank of the Sarasota block and continues southeast to the upper slope of the western Straits of Florida. The Sarasota Arch is a thick Paleozoic continental crustal feature overlain by a sedimentary section that probably consists almost entirely of shallow-marine carbonate rocks dating from at least the Early Cretaceous. The basement and overlying sedimentary section dip to the north into the Tampa Embaym.ent and also dip to the south into the area known as the South Florida Basin. South of the Sarasota block and along the western flank of the secondary block, the data indicate a downfaulted area presumably composed of block-faulted, subsided Paleozoic(?) crystalline basement and perhaps some Paleozoic-Triassic sediments. Although we have little age control in this area, we consider it reasonable to suggest that this subsidence and faulting activity can be correlated with Mesozoic rifting associated with the initial formation of the Gulf (Buffler, in press). We therefore expect that syndepositional Late Triassic-Jurassic non-marine through marine sediments and volcanic rocks immediately overlie this block-faulted surface and onlap and pinch out against the hanging wall blocks. The structural style of this sequence, here and on associated U T I G multichannel lines, is of multidirectional block faulting typical of passive margins (Harding and Lowell, 1979; Harding 1984). Considering this and the crystal thicknesses indicated by the OBS results, we interpret this central, downfaulted area and indeed the entire length of the outer platform to be underlain by slightly thinned, extended, rifted continental crust. Continuing to the southeast, t-here is a minor structural high termed the Sheffield Arch (Sheffield, 1978), which underlies and perhaps controls the location of a buried Tertiary shelf margin (Shaub, 1984). This feature is associated with a Bouguer gravity high and may include mafic intrusives and a series of faults involved in the middle Cretaceous foundering of the Florida Escarpment (Freeman-Lynde, 1984; Bryant et al., 1969; Mitchum, 1978). At the southern end, the Bouguer gravity values continue to increase under the continental slope. We denote as basement the 6.2-6.7 km s ~ arrivals that have a slight apparent basinward dip. In the adjacent deep Gulf basin, two additional refraction lines (not included here) indicate a crustal section substantially thinner and deeper than that derived from the platform lines (Ibrahim et al., 1981; Ebeniro et al., 1984). We therefore conclude that the crust thins westward from the platform toward the deep Gulf basin. We suggest that there may be no significant basement structural closure in the South Florida Basin since it appears that basement generally dips both to the west and south away from the large crustal blocks. The Sheffield Arch cannot be proven by our data set to be sufficiently large to produce an intrashelf basin with respect to the two major blocks.
CRUSTAl.
STRUCTURE
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SOUTH
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PLATFORM
379
7. Tectonic Interpretation T w o basic theories for the origin of the Gulf differ as to whether the composition of b a s e m e n t in the southern Florida Platform is oceanic or continental. Klitgord et al. (1984), for instance, suggested that this area u n d e r w e n t minimal lateral translation during the events which formed the Gulf, so that the area was subjected to a m a x i m u m a m o u n t of extension along transform trends with the resulting e m p l a c e m e n t of Jurassic-Early C r e t a c e o u s ocean crust. A c c o r d i n g to their model, b a s e m e n t here is primarily oceanic or nearly oceanic in composition. In contrast, Pindell (1985) considered the area to be a crustal block with c o n t i n e n t a l affinities, and suggested that this area was c o n t i g u o u s in the late Paleozoic with what is now the western Bahamas and formed a distinct block (the Florida Straits Block) which was transported intact to its present site from a location in the northeastern Gulf. T h e b a s e m e n t velocities from our data ranged from 5.8 to 6.7 km s -~ and fell into two clusters, one ranging from 5.8-6.1)km s -~ and the other ranging from 6.2-6.7 km s -~ (see T a b l e IV). T h e lower-velocity cluster included all data from Lines 2, 3, and 4 and from the s o u t h e r n m o s t OBS on Line 1; and the highervelocity cluster included all data from Line 5 and from the n o r t h e r n m o s t OBS on Line 1. G e n e r a l l y speaking, the interior part of the Florida platform was characterized by b a s e m e n t velocities we correlate with the Paleozoic b a s e m e n t of the Sarasota Arch; these correspond to c o n t i n e n t a l rather than oceanic velocities. F u r t h e r m o r e , we found no evidence for mantle arrivals in any of the data. We conclude from the observed velocity ranges and the absence of mantle arrivals that this region is more c o n t i n e n t a l than oceanic in nature. O n the basis of our limited data set, we propose that this area was originally underlain by continental crust. Moderate crustal extension and subsidence by normal faulting took place in the area to a degree that allowed the establishment and m a i n t e n a n c e of a mostly
Table IV Distribution of basement velocities Group
Velocity range (kin s ')
Basement
Line 1 Line 2 Lines 3, 4, 5
6.0-6.3 5.8-6.0 5.8-6.5
Basement velocity clusters
Lines 2, 3, 4 and Line 1 (OBS 3)
Intra-basement
6.2-6.9 5.8-6.0
Line 5 and Line ! (OBS 1)
6.2-6.7
All lines
6.4-7.5
Interpretation
Platform interior and basically continental. Platform margin and basically more oceanic.
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J O S E P H O. E B E N I R O ET AL.
shallow-carbonate platform margin adjacent to the deep Gulf area. Oceanic crust was indeed subsequently emplaced adjacent to our study area beneath much of the present deep Basin (Ibrahim et al., 198l).
8. Conclusion Until now, the deep structures underneath the Florida platform have eluded the seismologist. However, as the results of this experiment demonstrate, useful information about the regional crustal structure can be gathered using OBS refraction surveys. In summary, three structures were resolved: (1) the Sarasota Arch to the north, where the basement depth ranged from 3 to 4 km; (2) the South Florida Basin, which contained several sediment drapes just south of the Sarasota arch; the thickness of the uppermost drapes increased from 0.5 km near the arch to about 2.0 km at the center of the individual drapes; and (3) the Sheffield Arch to the south, which may be either an igneous intrusive or an uplifted basement fault block that partially determined the size of the carbonate platform. We observed that the most prominent and extensive group of arrivals characterizing all data sets on the carbonate platform had apparent P-wave velocities in the 5.5-6.0 km s -x range. Within this velocity range was the 5.8-5.9 km s ~ layer under Line 2 whose top was at an apparent depth of 3-4 km, in close agreement to actual depth of the crystalline basement under Line 2. The basement dipped both to the north (towards the Tampa Embayment) and to the south (towards the South Florida Basin) from the Sarasota Arch, and crustal velocities increased to 6.7 kms -I under the continental slope to the south. Deeper, higher-velocity intervals were also resolved. At no site were we able to detect any upper mantle arrivals. The lack of mantle arrivals and the magnitude and distribution of basement velocities under the south Florida carbonate platform is evidence that this region is part of the continental margin that surrounds the oceanic basin of the Gulf of Mexico.
Acknowledgements Dr. P. L. Donoho, designed the ocean-bottom seismograph units and served as Chief Scientist for the data acquisition cruise. We thank the captain and crew of the R~ V Ida Green for their efficient handling of the ship. We thank Dr. Yosio Nakamura for the helpful suggestions concerning the processing and interpretation of the data and Dr. J. Austin for his careful reading of this manuscript. Finally, we thank the following organizations for their financial support of the data acquisition: Chevron U.S.A. Inc., Cities Service Oil Company, Elf Aquitaine Oil and Gas, Exxon Production Research Company, Gulf Oil Exploration and
CRUSTAL STRUCTURE OF SOUTH FLORIDA PLATFORM Production Company,
38
Pennzoil E x p l o r a t i o n and P r o d u c t i o n C o m p a n y , Phillips
P e t r o l e u m C o m p a n y , Shell Oil C o m p a n y a n d U n i o n Oil C o m p a n y . T h e U n i v e r s i t y o f T e x a s I n s t i t u t e f o r G e o p h y s i c s c o n t r i b u t i o n N o . 663.
Reterences Antoine, J. W. and Ewing, J.: 1963, 'Seismic Refraction Measurements on the Margins of the Gulf of Mexico', J. Geophys. Res. 68, 1975-96. Barry, K. M., Cavers, D. A., and Kneale, C. W.: 1980, 'Recommended Standards for Digital Tape Formats', in Digital Tape Standards, Society of Exploration Geophysicists, Tulsa, 22-30. Buffer, R. T. (in press), "Seismic stratigraphy and Geologic History of the Deep Gulf of Mexico Basin', in Decade of North American Geology, Gulf of Mexico Basin, Geological Society of America. Burlier, R. T., Locker, S. D., Cagle, C. D., Sawyer, W. B., Crowe, J. C., and Phair, R. L.: 1984, Gulf of Mexico, Atlas 6, Ocean Margin Drilling Program Regional Atlas Series, Marine Science International, Woods Hole, Mass. Bryant, W. R., Meyerhoff, A. A., Brown, N. K., Jr., Furrer, M. A., Pyle, T. E., and Antoine, J. W.: 1969, 'Escarpments, Reef Trends, and Diapiric Structures, Eastern Gulf of Mexico', Bull Am. Assn. Pen.. Geol. 53, 2506-42. Ebeniro, J. O., O'Brien, Jr., W. P., and Chatterjee, S. K.: 1984, 'Crustal Structure of South Florida Bank Derived from Ocean Bottom Seismometer Refraction Profiles', Bull. Am. Assn. Pen.. Geol. 68, 473 (abstr.). Ewing, M., Woolard, G. P., and Vine, A. C.: 1939, 'Geophysical Investigations in the Emerged and Submerged Atlantic Coastal Plain, Part Ill: Barnegat Bay, New Jersey Section', Bull. Geol. Soc. Am. 50, 257-96. Freeman-Lynde, R. P. 1983, 'Cretaceous and Tertiary Samples Dredged from Florida Escarpment, Eastern Gulf of Mexico', Trans. Gulf Coast Assn. Geol. Soc. 33, 91-99. Harding, T. P.: 1984, 'Graben Hydrocarbon Occurrences and Structural Styles', Bull. Am. Assn. Pen'. Geol. 68, 333-62. Harding, T. P. and Lowell, J. D.: 1979, 'Structural Styles, Their Plate-Tectonic Habitats and Hydrocarbon Traps in Petroleum Provinces', Bull. Am. Assn. Pen.. Geol. 63, 1016-58. Holmes, C. W.: 1985, 'Accretion of the South Florida Platform, Late Quaternary Development', Bull. Am. Assn. Pen.. Geol. 69, 149-60. Ibrahim, A. K., Carye, J., Latham, G., and Burlier, R. T.: 1981, 'Crustal Structure in Gulf of Mexico from OBS Refraction and Multi-Channel Reflection Data', Bull. Am. Assn. Pert. Geol. 65, 1207-29. Klitgord, K. D., Popenoe, P., and Schouten, H.: 1984, 'Florida, a Jurassic Transform Plate Boundary', J. Geophys. Res. 89, 7753-72. Krivoy, H. L. and Pyle, T. E.: 1972, "Anomalous Crust Beneath West Florida Shelf', Bull. Am. Assn. Pert. Geol. 56, 107-13. Latham, G., Donoho, P., Grifliths, K., Roberts, A., and lbrahim, A. K.: 1978, 'The Texas Ocean Bottom Seismograph', Offshore Technology Conference Proceedings, OTC 3223, 1467-76. Maher, J. C. and Applin, E. R.: 1968, 'Correlation of Subsurface Mesozoic and Cenozoic Rocks along the Eastern Gulf Coast', A A P G Cross Section Publication 6, Tulsa, OK, 29 p. Martin, R. G. and Case, J. E.: 1975, 'Geophysical Studies in the Gulf of Mexico', in Nairn, A. E. M., and Stehli, F. G., (eds.), The Ocean Basins and Margins, 3, Plenum Publishing Corporation, New York, pp. 65-105. Mitchum, R. M., Jr.: 1978, 'Seismic Stratigraphic Investigation of West Florida Slope, Gulf of Mexico', in Framework, Facies and Oil-Trapping Characteristics of the Upper Continental Margin: AAPG Studies in Geology 7, pp. 193-223. Nakamura, Y.: 1983, 'Development of an Advanced Ocean Bottom Sensor System', Technical Report, Office of Naval Research Contract # N00014-77-C-0606, Inst. Geophys., Austin, 15 p. Pindell, J. L.: 1985, 'Alleghenian Reconstruction and Subsequent Evolution of the Gulf of Mexico, Bahamas, and Proto-Caribbean', Tectonics 4, 1-39. Purdy, G. M.: 1982, 'The Correction for Travel Time Effects of Seafloor Topography in the Interpretation of Marine Seismic Data', J. Geophys. Res. 87, 8389-96.
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Shaub, F. J.: 1984, 'The Internal Framework of the Southwestern Florida Bank', Trans. Gulf Coast Assn. Geol. Soc. 34, 237-45. Sheffield, F. C.: 1978, 'Where to Next in the Gulf of Mexico? A brief Review of Future Exploration Opportunities in the Gulf', Offshore Technology Conference Proceedings. OTC 3092, 383-90. Steinmetz, R. L., Donoho, P. L., Murff, J. D., and Latham, G. V.: 1979, 'Soil Coupling of Ocean Bottom Seismometer', Offshore Technology Conference Proceedings, OTC 3615, 2235-49. Sutton, G. H., Duennebier, F. K., and lwatake, B.: 1981, 'Coupling of Ocean Bottom Seismometers to Soft Bottoms', Marine Geophys. Res. 5, 35-51. Tatham, R. H. and Goolsbee, D. V.: 1984, "Separation of S-wave and P-wave Reflections Offshore Western Florida', Geophysics 49, 493-508. Warren, D. H., Healy, J. H., and Jackson, W. H.: 1966, 'Crustal Seismic Measurements in Southern Mississippi', J. Geophys. Res. 71, 3437-58. Whitmarsh, R. B.: 1975, 'Axial Intrusion Zone Beneath the Median Valley of the mid-Atlantic Ridge at 37~ Detected by Explosion Seismology', Geophys. J. Royal Astron. Soc. 42, 189-215.
