SOUTH TEXAS CONTINENTAL SHELF AND CONTINENTAL SLOPE: LATE PLEISTOCENE/ HOLOCENE EVOLUTION AND SEA-FLOOR STABILITY
(An outline and notes relative to geologic hazards on the sea floor)
U.S. Geological Survey
Open File Report
78-514
May 1978
Prepared by
Henry L. Berryhlll, Jr,
assisted by Anita R. Trippet TABLE OF CONTENTS
Page INTRODUCTION 1
THE STUDY PLAN 3 The systematic and integrated approach : 3 Rationale 3 Geologic components studied : - 3 Methods of study 5 Geologic framework < 5 Sedimentation > 5 Sampling methods and acquisition of data 5 Analytical techniques 5 Synthesis of data and reporting 12 Geologic characteristics of the region based on synthesis of the analytical and interpretative data < 12 Structural framework of the continental terrace 12 Evolution of the continental terrace since middle Pleistocene 15 General depositional history - 15 Holocene deposition 26 Tectonism 49 Relative stability of the sea floor 54 General classification of the shelf and slope - 54 Subarea 1 54 Subarea 2 54 Subarea 3 58 Analysis of the. unstable conditions in Subarea 3 59 LIST OF ILLUSTRATIONS
Page
Figure 1. Map showing bathymetry and topography of the South Texas Outer Continental Shelf 6
Figure 2. Map showing location of geophysical track lines, continental shelf - - . - 7
Figure 2A. Map showing location of geophysical track lines, continental slope - 8
Figure 2B. Map showing location of geophysical track lines, continental slope-- 9
Figure 3. Map showing location of bottom grab sample stations 10
Figure 4. Map showing location of core sample stations ^ 11
Figure 5. Map showing location of folds within the continental terrace ^~~ - , __*__ -, . ___-______,^, , _ j_j
Figure 6. Acoustical profile showing tensional faults in typical pattern above the crest of an anticline < 14
Figure 7. Acoustical profile showing tensional faults in typical pattern above the crest of a diapir 16
Figure 8. Acoustical profile showing deep seated rotational gravity faults in typical pattern, outer part of the continental terrace * 17
Figure 9. Acoustical profile showing composite of tensional and gravity faults, upper continental slope 18
Figure 10. Acoustical profile showing shallow seated rotational gravity faults associated with slumping of sediments down the continental slope 19
Figure 11. Map showing principal depositional environments, continental terrace off south Texas 20
Figure 12. Acoustical profile showing progressive growth of the outer part of the continental terrace by sediment buildup and progradation 22
Figure 13. Acoustical profile showing the continental slope seaward of the continental shelf with characteristic huramocky topography and slumped sediments 23 Page
Figure 14. Acoustical profile showing the nature and subsurface positions of two paleo surfaces that represent previous low stands of sea level, southern part of continental shelf - 24
Figure 15. Acoustical profile showing the nature and subsurface positions of the two paleo surfaces, reflectors A and B, that represent previous low stands of sea level, central part of continental shelf-r- 25
Figure 16. Map showing location of profiles shown on figures 17 and 18 27
Figure 17. Profiles showing stratigraphic relations of post middle Pleistocene depositional units : - 28
Figure 18. Profiles showing stratigraphic relations of post middle Pleistocene depositional units 29
Figure 19. Map showing configuration of the base of unit 4 (reflector D) 30
Figure 20. Map showing thickness and distribution of unit 4 31
Figure 21. Map showing thickness and distribution of unit 3< 32
Figure 22. Map showing configuration of reflector B, or base of depositional units 1/2 33
Figure 23. Map showing thickness and distribution of unit 2 = 34
Figure 24. Map showing configuration of the base of unit 1 (reflector A) ^ = 35
Figure 25. Map showing thickness and distribution of unit 1 36
Figure 26. Map showing interpretative environmental classifi cation of the types of sediments represented in basal Holocene deposits-- 38
Figure 27. Diagrams of selected cores, northern part of shelf, showing vertical variations in grain size *- 40
Figure 28. Diagrams of selected cores, central sector of shelf, showing vertical variations in grain size rr- 41
Figure 29. Diagrams of selected cores, southern part of shelf, showing vertical variations in grain size 42
Figure 30. Diagrams of selected cores from around an exposed Pleistocene reef showing vertical variations in grain size-^ ' = 43 Page
Figure 31. Map showing area of the shelf in which cores indicate
Figure 32. Map showing classification and distribution of surf icial sediments by grain size ; 46
Figure 33. Map showing percent sand in surf icial bottom sediments 47
Figure 34. Map showing interpretative contours of rates of sedi mentation in ram per year as determined by 210p^ activity in the sediments-- 43
Figure 35. Acoustical profile showing the subsurface position of reflectors A and B, stratigraphic reference horizons used to determine geographically the extent and intensity of faulting through time 51
Figure 36. Map showing geographic position of faults that cut reflector B but not reflector A 52
Figure 37. Map showing geographic position of faults that cut both reflectors A and B 53
Figure 38. Map showing .interpretative classification of stability, continental terrace 55
Figure 38A. Map showing a general outline of areas where "plumes" suggesting natural gas seepage were recorded on acoustical profiles 56
Figure 39. Map showing fault patterns, inner shelf in southern part of the South Texas OCS 57
Figure 40A. Acoustical profile showing structural and sedimen- tological characteristics of the shelf edge and upper slope off the Rio Grande delta 60
Figure 40B. Acoustical profile showing the slumped and faulted sediments on the continental slope 61
Figure 40C. Acoustical profile showing a combination of the unstable features shown by figures 40A and 40B< < 62
Figure 40D. Acoustical profile showing prograded undisturbed deltaic sediments overlying slumped and diapiric sediments at the junction of the continental shelf and the continental slope and slumped and faulted sediments on the upper part of the slop 3 63
Figure 40E. Acoustical profile showing slumped sediments-- * 64 Page Figure 40F. Acoustical profile showing both slumping and sliding on the continental slope - 65
Figure 40G. Acoustical profile showing slumps and slides on the continental slope 66
Figure 40H. Acoustical profile showing pile up of slumps at the base of the continental slope 67
Figure 41A. Acoustical profile parallel to the shelf edge showing prograded undisturbed deltaic sediments overlying slumped and diapiric sediments - 68
Figure 41B. Acoustical profile showing the continuation of undisturbed sediments above slumped sediments along the edge of the continental shelf - 69
Figure 41C. Acoustical profile showing a continuation of the undisturbed sediments over the slumped and diapiric sediments 70
Figure 42A. Acoustical profile, continental slope off south Texas, showing large scale slumping of sediments to con siderable depth below the sea floor 72
Figure 42B. Acoustical profile showing large scale slumping 73
Figure 42C. Acoustical profile showing large scale slumping - 74
Figure 42D. Acoustical profile showing large scale slumping 75
Figure 42E. Acoustical profile showing large scale slumping 76
Figure 43A. Acoustical profile showing the gentler gradient and deep seated rotational gravity faulting that is characteristic of the continental slope in the central sector of the continental terrace off south Texas 78
Figure 43B. Acoustical profile showing the characteristic gentler gradient and deep seated rotational gravity faults 79
Figure 43C. Acoustical profile showing large domal structure on the lower part of the continental slope caused by diapirism 80
Figure 44A. Acoustical profile showing slumped and deformed older deltaic sediments beneath younger prograded sedi ments and faulting associated with slumping on the continental slope (northeastern sector of continental terrace off south Texas) 81 Page
Figure 44B. Acoustical profile showing a rift caused by gravity slumping and pile up of slumped sediments, upper continental slope 82
Figure 44C. Acoustical profile showing characteristic hummocky topography above slumps and slides 84
Figure 44D. Acoustical profile showing characteristic hummocky topography above slumped and diapirically de formed beds 85
Figure 44E. Acoustical profile showing characteristic hummocky topography on the lower part of the continental slope ~ ~~ ______yjj
Figure 45. Map showing shallow geologic features of the con tinental slope caused by deformational movements within the continental terrace 87 SOUTH TEXAS CONTINENTAL SHELF AND CONTINENTAL SLOPE: LATE PLEISTOCENE/ HOLOCENE EVOLUTION AND SEA-FLOOR STABILITY
(An outline and notes relative to geologic hazards on the sea floor)
Introduction
In an engineering sense, the geologic aspects of the sea floor are
hazardous to man's activities when and where structures cannot be designed
to withstand the combined stresses of the marine environment. In the most
basic sense, geologic "hazards" relate directly to sea-floor stability,
or the potential of the sea floor for significant movement that would be
damaging to man-made structures placed on the sea floor. Judging the
potential for movement requires descriptive details about the geologic
conditions plus a quantitative understanding of the geologic and related
oceanographic processes that are operative in the region on both short-term
and long-term bases. Recognition and quantification of the geologic processes
and their implications regarding potential hazards must stem from an under
standing of the geologic history of the area in the recent past, which
requires determining the relative rates and interactions of the two basic pro cesses, sedimentation and tectonism, through time.
In an empirical sense certain features on the surface of the sea floor
suggest unstable conditions: irregular or hummocky topography caused by the
sliding or slumping of surficial sediments or diapiric movement, and off
sets of the sea-floor surface caused by vertical slippage or faulting.
The movements that created such features were a response or readjustment
to some type of instability; the factors that caused the instability may
have been localized or they may have been regional in scope. Furthermore, the movements indicated by the observed features may have been a one-time
event that reestablished equilibrium for a long time to come, or they may
have provided only temporary release of reoccurring stresses that
eventually will cause movement again.
Studies at two levels of detail are needed for the identification and
assessment of geologic conditions of a potentially hazardous nature in
areas where resource production is anticipated: a systematic study of
regional scope and in sufficient detail to describe properly the principal
geologic and environmental characteristics of the region and to indicate
the scope of their interactions; and follow-up studies as needed for those
specific sites that are in areas where the regional study indicates geologic
conditions are potentially hazardous. Topics of investigation covered by
the systematic regional studies should include: 1) the structural or
tectonic framework, with emphasis on the pattern and chronology of fault movements since the late Pleistocene, and the identification of tectonically active and structurally complex areas; and 2) aspects of sedimentation such
as surficial grain size relations, textural stratigraphy of shallow sub
surface sediments, extent, thickness and facies of the late Pleistocene and
the Holocene interglacial deposits, rates of sediment deposition during the
Holocene, surface expression of depositional features, and delineation of areas where the sediments have moved by sliding and slumping and where significant amounts of gas are indicated at shallow depth beneath the sea- floor surface.
The outline and notes that follow describe the geologic aspects of a multi- disciplinary systematic regional environmental survey made of the Outer Continental
Shelf off South Texas for the Bureau of Land Management that included biology, geochemistry, and physical oceanography. The outline proceeds from a short stability and potential hazards. Emphasis has been placed on illustrations which have been used liberally to document the geologic features discussed.
Investigators interested in a summary of the nature and results of the complete
environmental study are referred to Berryhill (1977) and Berryhill and others (1977).
Brand names of equipment used are capitalized throughout the text. The use of a brand name does not imply endorsement of the product.
The Study Plan
I. The systematic and integrated approach
A. Rationale
To investigate all components of the geological environment
in a unified rather than piecemeal manner so that the geologic
features and the processes responsible for them can be better under
stood and the interactions of the processes quantified to the extent
possible in time and space. The planned and integrated study approach
is based on the premise that the problem of analyzing and understanding
sea-floor stability is best handled by looking at all related components
of sea-floor geology simultaneously rather than topically and out of
context. Furthermore, in analyzing sea-floor stability in a predictive
sense, the past is the best key to the future.
B. Geologic components studied
1. Geologic framework (tectonic history):
a. Identify the internal structures shaped by crustal movements
within the continental terrace and determine their relations:
folds; faults. b. Determine the chronology of post middle Pleistocene
folding and faulting relative to the depositional
sequences identified.
2. Sedimentation
a. Determine the distribution of surficial sediments based on
grain size.
b. Determine the distribution of the sand-sized and coarser
fractions of the sediments.
c. Determine the stratigraphy of shallow subsurface sediments
to a depth of 2-3 m, with emphasis on the distribution of
discrete sand layers and the mechanisms of sand dispersal
over the shelf.
d. Determine the geochemistry of surficial sediments with
emphasis on key trace metals that indicate patterns of
sediment transport and the organic carbon content.
e. Relate the types of sediments to the physiographic sub-
provinces of the OCS and to localized topographic features,
f. Determine the rates of Holocene sediment deposition and
the thickness of Holocene sediments. g. Delineate to the extent possible areas of natural gas
seepage and areas where gas is suggested at shallow depths.
h. Document the post middle Pleistocene depositional history of the
region relative to sea level fluctuations caused by glaciation.
II. Methods of study
A. Geologic framework
High resolution acoustical profiling of two types in traverses
spaced to cover uniformly all physiographic subprovinces of the South
Texas DCS:
1. 3.5 kHz
2. 900 joule sparker
(See figure 1 for the physiographic nature of the region as revealed by
the bathymetry of the area and figures 2, 2A, and 2B for the spacing of
geophysical track lines used to provide data sufficient to describe the
geologic framework. )
B. Sedimentation
Sampling methods and acquisition of data:
1. High resolution acoustic reflection profiles: thickness of late
Pleistocene/Holocene sedimentary sequences; and history of sea
level fluctuations.
2. Coring: pipe and box coring for determining textural stratigraphy
and for identifying discrete sands in the cored sediments.
3. Bottom grab samples: grain size analysis; geochemical analysis.
(See figures 3 and 4 for location of bottom sample stations.)
Analytical techniques:
1. X-radiography an aid in core logging for identification of 27°00
97°00 96°00
Figure 1. Bathymetry and topography of the South Texas Outer Continental Shelf. 97-30'
ACOUSTIC PROFILE OBTAINED BY USE OF ACOUSTI- PULSE SYSTEM TRANSMITTING 900 JOULES. PLOTTED AT SEA BY PRECISION NAVIGATION fcSOFTJ
ACOUSTIC PROFILE OBTAINED BY USE OF SPARKER TRANSMITTING IO.OOO JOULES PLOTTED AT SEA BY PRECISION NAVIGATION C5OFT).
ACOUSTIC PROFILE OBTAINED USING TWO SYSTEMS: ANALOG REFLECTION SPARKER TRANSMITTING 9OO JOULES, AND SUB-BOTTOM PROFILER TRANS MITTING IOKW (3 5 rHj FREQUENCY) PLOTTED AT SEA USING A COMBINATION OF LORAN A AND HI FIX WITH A GENERAL ACCURACY OF TWO TO THREE LANE COUNTS(-3OO TO 5OO FT).
Figure 2. Location of geophysical track lines, continental shelf. <^z^ 1 65A !1
\ Ar^ ,1 - - I 67 U A- n loi 77 IN) \N» 1° A 1CD IcD ] 81 . - _ \ "T"" * T 91 A 92 11 \ L. IA 104 8 AM _ 1 3 Jp-l ^A 95°45' I3v T \ 1 26°30'4- A 1 17 1 FA 27 A 4"\ 32 4", 1
A 1 46 V A li 55 p A 60 £ A ILJ 67 /I fe A \° 71 vl0o- A 77 -.^IluJ A 82 A 90
96°00' I Figure 2A. Location of geophysical track lines, continental slope- 27
A A 70 72 A 80 A 100 104 A A 109 105 A 25, 122 01 A 128 A 01 142 100 A 147 145 X A 23 01 155 A 161 A 64 168 2\
A 173 /c \9 Al
A 16 17 19 A 34 / 15
27°00
13
101 101 01 101 Uee 11 (TO
96°00' A78 Figure 2B. Location of geophysical track lines, continental slope, 97°30' 97°00' 96°30' 96°00'
Figure 3. Location of bottom grab sample stations.
10 97° 30* 97°00' 96°30' 96°00' 95°30'
Figure 4. Location of core sample stations: black dot indicates core collected in 1975; circle indicates core collected in 1974; black square indicates closely spaced traverses across reefs.
11 depositional microstructures in cores and for precise definition of
tops and bottoms of discrete sand layers.
2. COULTER COUNTER for textural analysis of the less than 63 jun grain
size component of the sample.
3. Rapid sediment analyzer for textural analysis of the greater than
63 ;um grain size component of the sample.
4. Atomic absorption photospectrometer for analyzing the trace
metals content of the sediments.
5. LECO carbon analyzer for analyzing the organic carbon content of
the sediments.
C. Synthesis of data and reporting
All types of data compiled in map form to indicate spatial relation
ships; initially prepared on transparent base material so that all types
of topical data could be compared in overlay fashion to determine rela
tionships.
III. Geologic characteristics of the region based on synthesis of the analytical
and interpretative data
A. Structural framework of the continental terrace
1. Folds series of northeastward-trending folds identified as primary
structures within 'the continental terrace Csee figure 5 for loca
tion and trend of folds).
2. Faults five types recognized:
a. Tensional breaks formed in the strata above the crests of
anticlines (elongate structures) (fig. 6).
12 9 7'30'
10 IS 20 25 30 STATUTE MILES
60 KILOMETERS
I..,. ./;//,,, i. '
ANTICLINAL CREST SHOWING DIRECTION OF PLUNGE. OUTER LINES INDICATE 5 MILLISECOND CLOSURE AT 800 MILLISECOND SUBBOTTOM DEPTH; DASHED WHERE APPROXIMATE ONE MILLISECOND REPRESENTS 073m
ANTICLINAL CREST SHOWING DIRECTION OF PLUNGE. OUTER LINES INDICATE 3 MILLISECOND CLOSURE AT 800 MILLISECONDS SUBBOTTOM, DASHED WHERE APPROXIMATE
ANTICLINAL CREST SHOWING DIRECTION OF PLUNGE OUTER DOTTED LINES INDICATES INDEFINITE CLOSURE
MDNOCLINAL FOLD ARROWS INDICATE DIRECTION OF
DOMAL STRUCTURE; PROBABLY FORMED BY DIAPIRIC SALT
SYNCLINAL AXIS ARROW INDICATES DIRECTION OF PLUNGE
FAULT AT DEPTH TICKS INDICATE DOWNTHROWN SIDE
LEASE BLOCK AND NUMBER
Figure 5. Location of folds within the continental terrace.
13 ______< i < 1 \ WATER SURFACE
't **' ' =3
v-':
" : " -* TW - . _ ' ;:_ . 2
Figure 6. Tensional faults in typical pattern above the crest of an anticline. b. Tensional breaks above diapirs (domal structures) (fig. 7)
c. Deep seated rotational gravity faults in the outer part
of the continental terrace where sediment loading has
caused the rim of the continental terrace to sag and slide
basinward (fig- 8).
d. Composite tensional and gravity faults, edge of continental
terrace and upper slope (fig. 9).
e. Shallow rotational faults associated with sliding and slumping
of surficial sediments, continental slope (figs. 9 and 10).
B. Evolution of the continental terrace since middle Pleistocene
General depositional history:
1. Three general environments of deposition are recognized: open
shelf, deltaic, and continental slope (fig. 11). Each has
distinctly different sedimentary characteristics.
2. In general perspective, the outer edge of the continental
terrace has grown progressively seaward as a result of the
large influx of clastic sediments. The principal factor that
15 ^ji^i^^ 111
Figure 8. Deep seated rotational gravity faults in typical pattern showing direction of offset of beds, outer part of the continental terrace. Increased 1473 offset of beds with depth,-as indicated by A, B and C, indicates continued and progressive movement through time. 17 - ;:^.'-i>5:-^;;T^:j;^.;^j:;^ !^ll®|'-^\:Xr: i';i5'-:iS " ':> :>., I - .- - - J* . .- : .-'X- - :-:: Jl -- >..-: ' -
b:-.^ ft <-' ^ P- li :^1: ;
" " i ' -: '" :
-fe
\
AI04 * - r
1248
,--!-. .: -.! L . f 111:; 1 Figure 9. Composite of tensional and gravity faults, upper continental slope. 0937Z BEGIN STARBOARD LOOP
WATER SURFACE
-. /' -'---'' ' .- - " ;' 7- ; "» ' 3"^ - - '-'.- -'- - "- ; "-"" - - -'-" ."- - "- t/> i--".. fe&tv '-''; ^'!"-';''--:-".--' :''^
Figure 10. Shallow seated rotational gravity faults associated with slumping of sediments down the continental slope.
19 28°00'
27°00'
97°00 96°00
Figure 11. Principal depositional environments, continental terrace off south Texas. (A = ancestral deltaic; B = open shelf; C = conti nental slope)
20 has shaped the geomorphic profile of the terrace has been the
climatically controlled fluctuations of sea level. Sea level
has risen and fallen cyclically over a range of some 100 to
150 m during Pleistocene time. The nature of the general
depositional process is shown by figures 12 and 13, which are
adjoining sections from an acoustical profile. The development
of the continental terrace in the western Gulf has been discussed
previously by Lehner (1969), Curray (1960), Moore and Curray
(1963), Sidner (1977), Sangree and others (1976), Tatum (1977),
and Pyle and Berryhill (1977).
3. The geomorphic extent of open shelf versus deltaic deposition
has varied widely because of the changes in sea level: sedi
mentation has been principally deltaic during the glacial epochs
when the shelf was exposed as land surface and open shelf during
the interglacial epochs when the sea rose and spread landward
across the shelf. Deposition has been continuous over the slope,
but the amount and type of sediment deposited there has varied
considerably as the result of the migrations of the shoreline.
4. Previous low stands of sea level are represented on the
acoustical analog records as strong reflecting surfaces that
are irregular and nonconformable to the sediments above and
below. Figures 14 and 15 are sections of acoustical profiles
from different parts of the shelf showing the nature and
21 CONTINENTAL SHELF
a^wc^jwsffS' '*""' " :&3&
r/.'l**;£ J^t"' *.: ; *S8B £*ffi?« S,^.^^JS*S;a &&
A55 A56
______.-.- ._.._- ,^_. _____.- .-- ....-.-.. .^.,__ .-.-. « =-. -,.^.,«--« ..- o-m------"t'>-H' 1248 ^ Figure 12. Section of acoustical profile from the southern part of the south S 1300 >^» Texas outer continental shelf showing progressive growth of the outer part K ^;-; of the continental terrace by sediment buildup and progradation: numbers 1 $' CONTINENTAL SHELF r
-" - i^' rt!^--....,.';"1 ^ K;&'*' ; -: ' - ' U--: " ^^.'%J^%\i^^ .'-: :;;:
^S^m^fysK*JT?:J?- -^-.-.Vf.w"-^. ^ti>JA 'i*-£Sfc^ , ^1- OC-A ci r»r»o $ -
Figure 13. Section of acoustical profile adjacent to that in figure 12 showing the continental slope seaward of the continental shelf with its characteristic hummocky topography and slumped sediments. WATER SURFACE " n?- 40
5 EROSIONAL CHANNELS^
==ggy^-g?ir3sg2S3=2s
^!?'fe^;£=*
^£±-^^>e^^p^^-^^g?^^^:^r:
Figure 14. Section of an acoustical profile, southern part of the south Texas continental shelf, showing the nature and subsurface positions of two paleo surfaces that represent previous low stands of sea level.
24 3,000 ft. 4 ; 4 ^TW-TV -.- -.#.. # * -'Sf^ H SEA FLOOR >'-: ii--c^^v^^:x'---~^tC2*''?«D C" c i troT/^D A !££ fgy^a^Vfgft^sstn_L r L t L I UK A r£z
^^^^^^^^S^^S^^^^^^^^^-tj^^^^^^^ "S*^ B
?&$£*£&£ &+
rr^2crc
A72
// - -.-.. -
that represent low stands of sea level.
5. The unconformities represented by the strong sound reflecting
surfaces define 4 late Pleistocene/Holocene depositional units
on the shelf. The stratigraphic relations of the depositional
units and the unconformities separating them, as constructed
from acoustical profiles, are shown by figures 16, 17, and 18;
figure 16 is the .index map showing the locations of the profiles
shown on figures 17 and 18. The distribution and thickness of
the units, in essence the depositional history of the shelf for
the time period specified, are shown by the series of isopach
and structure contour maps, figures 19 through 25 taken from
Pyle and Berryhill (1977). The isopach maps show the thickness
of the units; the structure contour maps show the configuration
and depth below sea level of each of the unconformities that
define the depositional units.
Holocene deposition:
1. The Holocene deposits record the transgression of the rising
sea across the shelf from the last low stand. The older
Holocene deposits making up the basal part of the unit
probably represent different environments from the more
recent open shelf deposits. An interpretation of the types
26 97°OO' 96°00'
20 40 60KM
Tigure 16. Location of profiles shown on figures 17 and 18. (From Pyle and Berryhill, 1977.)
27 TWO-WAY TRAVEL TIME (msec) ro ro ro o o o o o o o o o o o o I 1 a> ro H- M XJ OQ i-f O C
en H- " (W
O p* Hi H- H- O M 0) H W h-0) 1 > (13 " rt H- W O p 03 Cfl
n
a.
m _ 70 C ? m m 2 m CD m H o m TI CO O O CD >
CO PROFILE D
SEA FLOOR 100-
200-
o PROFILE E Q) o
LU
100- LU
\-a:
2OO -
PROFILE F 0
100-
200-
Figure 18. Stratigraphic relations of post middle Pleistocene depositional units, profiles D, E and F. (From Pyle and Berryhill, 1977.) See figure 17 for legend.
29 97°00' 96°00'
10 20 30NMI
CORPUS CHRIST/ CORPUS CHRISTI
STRUCTURAL CONTOUR BASE OF UNIT 4
UPDIP LIMIT OF U PORT UNIT 4 MANSFIELD SEAWARD EXTENT OF TRUNCATION BY REFLECTOR D REEF SHAPED MOUNDS
PORT ISABEL..-
Figure 19. Configuration of the base of unit 4 (reflector D) shown by structure contours in depth below sea level in milliseconds. One ms is assumed to represent ~0.73 m. (From Pyle and Berryhill, 1977.)
30 97°00* 96°00'
10 20 30N Ml
Figure 20. Thickness and distribution of unit k in milliseconds. One ms is assumed to represent -0.73 m. (From Pyle and Berryhill, 1977.)
31 97°00* 96°00*
10 20 SON Ml
OBLIOUE-PROGRA DATIONAL FACIES
Figure 21. Thickness and distribution of unit 3 in milliseconds. One ms is assumed to represent -0.73 m. (From Pyle arid Berryhill, 1977.)
32 97°00'
STRUCTURAL CONTOUR REFLECTOR B
UPDIP LIMIT OF UNIT 2 - BASE OF UNIT IS MASKED BY BUBBLE PULSE SUBCROP OF UNIT3
SUBCROP OF UNIT 4
SUBCROP OF MA TERIAL OLDER THAN UNIT4
' ^°nfifration of reflector B, or base of depositional T* Y structure contours in depth below sea level
33 97°00' 96°00'
10 20 SON Ml
28° 00'
27° 00'
Figure 23. Thickness and distribution of unit 2 in milliseconds. One ms is assumed to represent ~0.73 m. (From Pyle and Berryhill, 1977.)
34 97°00' 96°00'
10 20 30N Ml
28° 00'
27° 00'
STRUCTURAL CONTOUR _ PORT BASE OF UNIT 1 MANSFIELD BASE OF UNIT IS MASKED BY BUBBLE PULSE BASE OF UNIT 1 IS DEFINED BY REFLECTOR A
QL= 10msec PORT ISABEL^
Figure 24. Configuration of the base of unit 1 (reflector A) shown by structure contours in depth below sea level in milliseconds. One ms is assumed to represent ~0.73 m. (From Pyle and Berryhill, 1977.)
35 97°00' 96°00'
10 20 30N Ml
28° 00'
27° 00'
- PORT MANSFIELD ISOPACH UNIT i
- BASE OF UNIT IS MASKED BY BUBBLE PULSE A A' PROFILE LINE
Cl= 5 msec PORT %\M ISABEL^ I ______
Figure 25. Thickness and distribution of unit 1 in milliseconds. One ms is assumed to represent ~0.73 m. (From Pyle and Berryhill, 1977.)
36 of sediments represented in the basal Holocene deposits is
shown by figure 26.
2. The overall thickness of the Holocene sediments is related to
the structural fabric of the continental terrace; the structural
grain influences both the directions of sediment transport and
the sites of deposition (see thickness map for unit 1, figure
25).
3. The nature (composition and grain size) of most of the Holocene
sediments is not known from actual observation, but the seismic
records suggest that, except for the basal deposits that seem
to vary, the makeup probably is similar to that of the shallow
subsurface Holocene deposits penetrated by gravity cores. The
textural stratigraphy typical of the shallow subsurface sediments
is shown by selected cores (figs. 27 through 30). The several
depositional environments of the shelf are represented by the
cores: inner shelf, mid shelf, outer shelf, and relict deltaic.
(For locations, see figure 4.)
4. Thin discrete layers of sand are typical of the shallow sub
surface deposits over the inner half of the shelf. The
stratigraphic recurrence and wide geographic distribution
37 Figure 26. Interpretative environmental classification of the types of sediments represented in basal Holocene deposits. The .heavy broken line is the postulated position of shoreline during the last glacial (Wisconsin) epoch. The present position of shore line is shown by the short dash/dot line. See next page for legend,
38 EXPLANATION
Deltaic
Principally prograded sediments; shelly sand predominates to the depth cored. Ruled pattern indicates area where youngest deltaic sediments are prograded over underlying slumped and contorted sediments.
Fluvial
Lobate sheet of coalesced fluvial deposits containing numerous buried stream channels.
Mixed
Localized fluvial deposits resting on older open shelf deposits Area east of dashed line probably contains lagoonal deposits.
Lacustrine?
Area outlined on the basis of the acoustical properties of the sediments. Almost complete attenuation of sound suggests either a large amount of shallow gas or high organic content.
Carbonates
Isolated reefs surrounded by irregular thin sheet-like deposits that are believed to be reef debris and small carbonate mounds on the basis of high acoustical reflectivity. Black dot indicates reef exposed on sea floor; irregular shaped circle around the dot repre sents general extent of the reef platform buried by Holocene sediments 39 26 25 23 22 CM 0- ......
C.\J
'."".**' 40- y/x.-v
60- """"' *
80-
100-
ft
44 43 28 41 40 39
Figure 27. Selected cores, northern part of shelf, showing vertical variations in grain size. Station numbers are at top. Locations are shown by figure 4. Stippling indicates sand; no pattern indicates mixed silt/clay. Comma-shaped marks represent shell remains. Column at right indicates degree of textural modification by bioturbation as determined by Hill, 1976, 1977: full pattern >60%; half pattern <60%.
40 145 146 149 141 155 CM 1 O-i -*^>*_ 10- '. "I; \V " >';. ^ m 20- I m m 30- ^ m m 40- m 50- m \- 60- m o 70- ,101 -I 80- E3 90- §1 m 100^ Em m 03 m n m ' i *T m m 03 m m G3 m m H ***3 m H m 03 ED -ix£Z
165 164 161 160 £ii i *g m m m m ~~~ m m m m *5=£ m m m m m m m m m m m m m m m m on m on m m m m tfZKQ] m m I Figure 28. Selected cores, central sector of shelf, showing vertical variations in grain size. Station numbers are at top. Locations are shown by figure 4. Box and piston cores from same station shown. Sand indicated by stippling; lines indicate coarse silt; no pattern indicates mixed silt/clay. Crescents represent shells. Column at right indicates degree of textural modification by bioturbation as determined by Hill, 1976, 1977: half pattern <60%; square <30%. 41 204 206 223 221 220Q 228 CM 0- <;: ?0- i-rrrrri < ^*; - :'v5.:: #" 40- ! ! 11 60- ??i?: ifc (#£ Qf\ ^>
::::::: 100- C : ::::::: If ::::::::: :::: ::: : ?::: * :^| ; c ': ~ m !=i * Oyster shell MY" ::::::::
:=:::::
=;;==;;::=|: :::: ::: ^ ...... ::: ;=;= =; ;;;= -H
:V-^.: Dhione Clench! Bay, shallow iHliilli!!! surf form
*Very even-grained * - eolian-type sand
::::: :::::
226a 248 251
_LU^_, ll-gr-iiH
'«'# ':: f< li ")" * * "c" V" e ., 0-J-." . ~ j -c- -t- -'-- - t-> -j_; ; , : - . * >* :'e:'i:: * Very even-grained ::::: ; ::: eolian-type sand *?M -V: . >.- ** Hard brown clay ;;;;I=;HH with carbonaceous &? plant roots -"*:/ . ; cVc-o t.ct. ::::::::::-.: Figure 29. Selected cores from the ancestral Rio Grande delta, southern part of shelf, showing vertical variations in grain size. Station numbers are at top. Locations are shown by figure 4. Stippling indicates sand; no pattern indicates silt/clay. Comma-shaped symbols represent shells; black represents large amount of carbonaceous mud. Column at right indicates amount of textural modification caused by bioturbation, as determined by Hill, 1976, 1977; full stipple >60%. 42 CORES AROUND HOSPITAL-ARANSAS REEF 79 80 85 1 i CMo- * ^^ 1 10- i 20- ^ <* )| 30- i: 40- A < E3 J 2> 50- E3 60- 0 ^ 70- HO 80- ^ 90- '.'..'*.' ' i 0 100- EJ HO ED H) n 13 HD 013 Si H *J ED H n El E3 d l^n " us rii»- ^s**" m n Figure 30. Selected cores from around an exposed Pleistocene reef showing vertical variations in grain size. Stippling indicates sand; no pattern indicates mixed silt/clay. Crescents represent shells. Column at right indicates degree of textural modification by bioturbation, as determined by Hill, 1976, 1977: half pattern <60%; squares 43 of the discrete sands suggest periodic relatively rapid water movement or high energy conditions over large parts of the shelf. Hurricane passage may be the principal mechanism by which the sand is distributed beyond the surf and shoreface zone. The area of the shelf in which discrete sands occur is shown by figure 31. 5. Distribution of surficial sediments by grain size indicates that silt is the predominant textural component (Shideler, 1976; 1977). Relict deltaic and ancient shoreline deposits of sand are on the southern and northeastern parts; otherwise the variations in grain size regionally are what might be expected on empirical grounds: increase in the sand-sized fraction shoreward and increase in the clay-sized fraction seaward. The distribution of the surficial sediments by grain size and by sand content are shown by figures 32 and 33; the distribution of the shallow subsurface sediments by grain size is similar to the surficial sediments. The shelf as a whole is a region of typically fine-grained sediments. 6. Dating of the rates of sedimentation during the past few hundred years using XJ91 -0u Pb as the dating agent, indicates that the highest rates of sedimentation have been at midshelf in the northern part of the South Texas OCS (see figure 34, 44 28°00' 27°00' 97*00 9«*00' Figure 31. Area of the shelf in which cores indicate discrete sands; possibly spread in the aftermath of hurricane passage. »7'30' SEDIMENT DISTRIBUTION MAP (Shepard's Classification) - Sand 6 - Sansicl 2 - Silty Sand 7 -Clayey Silt 3 - Sandy Silt 8 - Sandy Clay 4 - Silt 9 - Silty Clay 5 - Clayey Sand 10 - Clay Figure 32. Classification and distribution of surficial sediments by grain size. From Shideler, 1976. 46 96*00' SAND PERCENTAGE ISOPLETH MAP Figure 33. Percent sand in surficial bottom sediments. From Shideler, 1976, 47 RATES OF SEDIMENTATION 26° MEX:\U.S.A. , ..^ I I I Figure 34. Interpretative contours showing rates of sedimentation in mm per year as determined by ^10Pb activity in the sediments. From Holmes and Martin, 1977. 48 from Holmes and Martin, 1977) . Comparison of figures 34 and 25 demonstrates the northward shift of the Holocene depocenter in latest Holocene time. 7. Distribution of trace metals in the surficial bottom sediments indicates two relationships relative to sediment grain size: increased abundance in the finer grained sediments; and increased abundance around certain estuaries where anthropogenic induction is suspected. The use of barium content of the sediments in conjunction with ^lOp^ dating indicates that sediments are being carried primarily from north to south and obliquely across the South Texas OCS. The barium content of the sediments began to increase in the 1920 f s as oil well drilling increased. Thus, it is an ideal tracer element for measuring rates and patterns of sediment movement. Tectonism: 1. Most faults are of the growth type; relative offset along the faults increases downward with increasing subsurface depth, indicating continued movement through time. Growth-type faulting is well illustrated by figure 8. Note the increased offset with depth along individual faults. 49 2. Faulting, though relatively continuous over the broader span of time from the middle Pleistocene to the present, appears to have been episodic when viewed in shorter increments of time, possibly as a result of the cyclicity of the alternating periods of glacial sea level withdrawal and interglacial depo sition. The faults have been plotted relative to two of the subsurface reflectors already discussed to determine both the geographic patterns of faulting and the variations in the intensity of faulting with time. The depths below the sea floor of the two subsurface reflectors (equivalent to reflectors B and D respectively, in figures 17-25) is shown by the section of an acoustical profile, figure 35. The geographic locations of faults that cut reflector B but not A are shown by figure 36 and those that cut both A and B, by figure 37. 3. Comparison of the two maps shows that the faulting has migrated seaward across the shelf during late Pleistocene/Holocene time. 4. Faulting during the Holocene has been concentrated along the outer edge of the continental shelf. 50 /$ WATER SURFACE ** '" w I-. . I -.- O _l . 1 OJt REFLECTOR A Pi^r-^r- **i-^r* v ' '^ --^-"-^ v- * >C^''^- ^t-cU^-'i'^*"? l~.r kr?*---l^^^i^\-£^^C^jV*^^.^^-^ -^t^T-.-v^i- -l-^.- ^^-^' V ^^ ."'.^. ) ,"^~< *:f^*±:.} ?rA *^~ '.-^ Figure 35. Section of an acoustical profile showing the subsurface position of reflectors A and B, stratigraphic reference horizons used to determine geographically the extent and intensity of faulting through time. 51 97°00 96°00 0 5 10 15 20 25 30 N Ml 0 10 20 30 40 50 60 K SAN ^ANTONIO BAY CORPUS CHRISTI CORPUS -:A BAY CHRISTI Figure 36. Geographic position of faults that cut reflector B "" but not reflector A. Plotted from acoustical profiles. Tick indicates direction of offset along the fault. Stippling indi cates slumped sediments now covered by younger non-slumped sediments. f-A PORT 1 MANSFIELD PORT ISABEL 97°00 96°00 0 5 10 15 20 25__30NMI 0 10 20 30 40 50 60 K CORPUS CHRISTI Figure 37. Geographic position, of faults that cut both reflec tors A and B. ^lotted from acoustical profiles. Tick indi cates direction of offset along the fault; circles indicate faults that extend to the sea- 7 floor surface. Stippling indi cates slumped sediments. LL 7 IV. Relative stability of the sea floor A. General classification of the shelf and slope (figure 38) Subarea 1 1. Generally stable. No movement apparent over most of the inner half of shelf in post middle Pleistocene time, but in the southern part of the DCS recent faults cross the shoreline from shelf to coastal plain; the faults are expressed at the sea floor as topographic ridges (see fig. 39). 2. Movements at mid shelf have decreased during the late Pleistocene, but localized fault movement along the crests of folds is indicated. At isolated sites, faults reach the sea floor (see fig. 38). Subarea 2 1. Generally stable but faulting has been both extensive and intensive over the outer third of the shelf in the central sector of the DCS during the Holocene. 2. The faulting along the outer shelf has been caused by the combination of sediment loading and isostatic adjustment; subsidence has been countered to a degree by the buttressing effect of a rising anticline whose crest lies just beyond the shelf edge beneath the uppermost part of the continental slope. Figure 8 is a section across the outer shelf and upper slope showing the structural relationships. 54 T'»0' Figure 38. Interpretative classification of stability, continental terrace: s-ubarea 1, generally stable except for isolated sites indicated by symbols where faults reach the sea floor; subarea 2, outer shelf where faulting has been extensive during the Holocene; and subarea 3, the continental slope where slumping and diapiric movements have been extensive. 55 Figure 38A. General outline of areas where "plumes" suggesting natural gas seepage were recorded on acoustical profiles. 56 I V-'^Av^ ir\ y*\\ \\ I \\ i \ "X.\\ \ \ \\ l . I / * \/ V \ \ \ V»\ i *^\\ * v \ '1 ! /» *» f*\ \\ \:-\ $\ ,*"> \ \ ' *» \]L\\i «' ' \ \ \/ V \ ' ^ ^ 'v l$it£& Xi l Wtp?.i/ ,f \ ftSji-LiKt^ lv,\; v\ Figure 39. Fault pattern, inner shelf in southern part of the South Texas DCS. Note the association of some of the faults with the elongate topo graphic features, indicating the relatively recent age of the faulting. 57 3. Gravity slumping of sediments and associated shallow faulting is evident in the southern and northeastern parts of the OCS near the shelf edge and on the adjacent uppermost slope. Subarea 3 1. The continental slope has been a mobile area and relative instability is indicated. Mobility has been primarily of two types: massive, large scale gravity slumping of relatively soft sediments down the increased topographic gradient of the continental slope; and diapiric movement of salt. The charac teristic hummocky topography of the slope surface is expressed by the bathymetry map, figure 1. 2. Some of the rotational, gravity faulting on the slope does extend to considerable depth in the subsurface, indicating the tendency for the entire outer part of the continental terrace to slide toward the deeper central part of the Gulf of Mexico because of sediment outbuilding and loading. 3. The relative instability at the shelf edge and on the con tinental slope is primarily a function of the increased topographic gradient which in turn relates to the adjustments caused by sediment buildup and overloading. 58 B. Analysis of the unstable conditions in Subarea 3, shelf edge and adjacent continental slope, by geographic segments from south to north: southern, central, and northeastern 1. Southern a. Geologic characteristics: edge of ancestral Rio Grande delta coincides with shelf edge; increase in sea-floor gradient; slumped and diapirically deformed sediments of a generally chaotic nature lie beneath younger prograded and undeformed sediments; shallow faults are associated with surficial slumping and sliding; deep-seated gravity faults are associated also with broader scale basinward slumping, b. Types of instability and potential hazards: (Examples shown by 2 sets of figures that are adjoining sections of acoustical profiles: numbers 40A-40C; 40D-40H). (1) Buried older slumped and chaotic sediments beneath prograded undisturbed sediments. Potential for over- pressured zones and high gas content. Examples across strike or down the slope are shown by the two adjoining sections of an acoustical profile shown in figures 40A and 40B and also in the single section, figure 40D. Examples of the same type of features along strike or parallel to the edge of the shelf are shown by figures 41A, 4IB, and 41C. (2) Surficially slumped and faulted sediments along edge of shelf/uppermost slope (figs. 40A, 40C, and 40D). (3) Primarily surficial, shallow seated sliding and some slumping on the upper to middle slope (adjoining 59 STEEPLY DIPPING -*^,) *» i *.dL*P -*»>* . * Figure 40A. figure 2A, teristics Grande delta. prograded r~>£ Figure 40B. Section of an acoustical profile adjoining'; on the east that in figure 40A showing the slumped and ^ I £>5XftCV*W faulted sediments on the continental slope. ^ ^^JK ?Mm; v.1^^5^^I ^^^ A 41 A 42 61 - K . .% »./.!,«i.. !" ;; '.. . ' - :. ,-u-rvr '.. f, .-.. . r-vv^ -,; - . ,-:r ".- Figure 40C. Section of an acoustical profile showing a combination of the unstable features shown in figures 40A and 40B. .fe MIM >> <"V», "T/'-'t -.-liigfiiii >#> ' ':*: £$; '^f -'', *v» iffii* >\» > '* V^<^/^^^^;^Y:^':' ??^vl^»^ mmrnm Figure 40D. Section of an acoustical profile along line AA, figure 2A, r.^T^ showing prograded undisturbed deltaic sediments overlying slumped and 'O^- diapiric sediments at the junction of the continental shelf and the ^.''< ' :.'<-; Figure 40E. Section of an acoustical profile downslope and continued r ^>^y^:-'.:\v - Figure 40F. Section of an acoustical profile downslope and continued \ ' / from figure 40E showing both slumping and surficial sliding on the continental slope. './no. 290 '/',; MOST RECENT :OLDER,BURIED :,r; ~P^fe3£jx&$ i£-V-U*r?f?.j:5r" ^ 332^J^%i^.'. .-.- i T7T . ??*ii.'lTr-? . H" Figure 40H. Section of an acoustical profile downslope and continued '. ' from figure 40G showing pile up of slumps at the base of the con ^ r" tinental slope. V- ' « . v» ^1. .,'';,.'; ' : '-'.;;'? -v.'- ' ( . '« :-, ... " :-.- '., ; ; , . £ .. ;:| .: : 2000ft. i-^-r; :^3 I '>..":' .'.». '.' ' ', :-^\'<-.' *' -:.:'-?.* ;: ' ;"-*'". ' . ' &-'.(' <*i !t ' '' .' ' ' . ' ...... ^ , . _. . _...... _.^ " j£jgi4i3li SEA FLOOR - " 7«>v r^'./vrit-^1"' »-:-f^:^ .rr.---^i:-^'-ri:4:''.r lj-'<^1.* ^Sr^: ' i'.£?*j C-M-C; v'.^>:' ?^Xr:-f' . *-J.M. .S'.':t*Aie?-v<"'»*iJ.1 :tr*-^ri^^t ^.^i, . '^''l&TVl ; £^^>'3<^fe'-' ,.«-iJ'V 'li I' -^ '..., 7 r.1^ i^^^^ii^r-*^?-'^..'*-' :...-^.: "-.*j'n, r, >»v^ r , , w-> '*»;..'vu-1- .-''1 '//'- /' ""-.."V *** '''^. J^;g|^rp[ i^-^v^^^ -;*>-vV'''''''"v''1' &^ ;^^ y.-"1 C'r-'^'i.-.:>?;v.\i;<: 12 ,A"/.>- ',>>«A ' / "fxjw^." »-ar»; lvVf*WV ' ",V ^i-rrrx-'.W *''» / '* '< , 2 '*i?f+£$?$ '"'- pX»f>> <\* ^v^-;-^^ r- *^S- >»rKV£ ' ' I-- L:< : '. -35 v>k*>; U^^^1: '^%^ / ^^?'< Figure 41A. Section of an acoustical profile parallel to the shelf edge 8 #*?£> showing«u«..,j prograded j-j undisturbed-.-t. . , , deltaic, , . sediments overlying slumped and rl-foi-4-}-!--!^ <-t /-iJ -?«. «.^ *. _ A89 *as«aort«>acB»i« rc^'^>»>^! no. 4 Sr^^^f^^^^^g^^?^ "no. 6 ^HT^^^4^?^^ '~ . ~ ' '" .,,£' it*. >-''»\'.V.'?*'~''\ ' *?;'j£ %'££X~-£rir*i'!^:*rt'n'~y ^---"* r ',.* " ,:',i-i?y^!; "'?. ' ". ^^j^Jvij*-' 5:^x.rv--j^-*.; i">£''*-»yV'" » 1 **^» . V ^«'V<' "-^^ -<^ ~ -- .^l??!?^ 2- - l.7i"i;^--'"^' « -.- .>» W, 'uj»>i'',reS^Cl~.-'-r- -* y » »*.v*^ii»i »^. .-_ " ">" V^»^- > ,"7.i T."*1 n7:rTi^rf iiiii yv~ii'.uja "V.** =c>^^3^S^S»^ .:c^^.rT-'-^-'*«=r!S^5=&1c :r«#s«r^« sjjs^i^^j «^V« ,^«r. i- - v:-'rr;; j -, ! xj.- . .?.>--,- vxi ^£^z^££z£-i^z^^^ ^^^^^^i^^^^ ^^^ - «^-;.'>.-.V' s:*,!«/*>,...7- - <^5 v>, v »''> "** X&M x/'.>>k' ' '^.^iSjrisi. T/V*/:^ Figure 41B. Section of an acoustical profile adjoining on '«».' i' >» : ,!'; ' ,-'I. -.'! V ;;» .' ;,: . '. ' / ' I. * i' ' ii . < ,' !| no.84 :. ' . , |£< ijl no' 87 i ; ''-'^ ' "'I' '! . ' . . ' : no. 90 i " I - t ' : '. >.'*'^ 4^(#Ff it-3esEs§8 r - « 2000ft. :,«>; ' . » ; . ' ' ''.. . ;&u SEA FLOOR Figure 41C. Section of an acoustical profile approximately 10 km north v*Jl of that in figure 41B showing a continuation of the undisturbed sedi 3 d ments over the slumped and diapiric sediments. Note the lateral flowage of sediments near the left hand edge of the section. i figures 40F-40H). Compare the two sets and note the difference in the type of deformation. (4) Large scale slump/folding and faulting on the middle to lower continental slope involving sediments to considerable depth below the sea-floor surface (adjoining sections, figures 42A-42E). 2. Central a. Geologic characteristics: subsiding edge of shelf being countered by rising anticlinal ridge that has isolated diapirs along its trend; many deep seated rotational faults and tensional faults; increase in topographic gradient at shelf edge less abrupt than to south and north and no previous surficial sliding or slumping of the sedi ments on the upper slope is indicated. Gas seepage in association with faults is indicated over the general outer central sector (fig. 38A). b. Types of instability and potential hazard: (1) Active anticlinal fold beneath the upper continental slope. (2) Active diapirs: two along the anticlinal crest that lias beneath, the -upper slope and at least 6 large 71 . if v- i'*i ** jvr* ,'*'«",'/' f *;-. 7, n r \ TA*J ^i-*"* * r '" J ' / «* r *Mi**;" '"'"** l / ^' ^ Figure 42A. Section of an acoustical profile, continental i'.'*"'. slope off south Texas showing large-scale slumping of sedi- y. ,';' ments to considerable depth below the sea floor. VTfVu >«75k. " ' ' T "~l "?*- ir, '* i v" mm, IBIfiit '' 'v' :'*' 15; *Uf4i' ';- affigfflttfe^ diapiric masses further seaward beneath the middle and lower slope. Characteristics of the outermost shelf and the adjacent continental slope in the central sector are shown ,by figures ASA, 43B, and 43C; upper slope, 43A; mid slope, 43B; and diapiric mound on lower slope, 43C. 3. Northeastern a. Geologic characteristics: edge of ancestral Brazos-Colorado River delta coincides with shelf edge; increase in topographic gradient at shelf edge relatively abrupt in contrast to the central sector; slumped and diapiric sediments lie beneath prograded younger sediments; shallow faults associated with surficial slumping; hummocky topography caused by slumping on the continental slope, b. Type of instability and potential hazard: CO Slumped and diapirically deformed older sediments beneath prograded younger sediments (fig. 44A). (2) Slumped and faulted sediments beneath the steep upper part of the continental slope (fig. 44B). (3) Relatively steep and irregular bedding in prograded sediments along edge of delta (fig. 44A). 77 ... :< ».:.. .I 1:;:!;- .5::;-! ;'-'? l. ] :\. -v , no.40.fj, : '. ;;./jV^.i|.'-.i . ; :: : - :i --i.- :.i !:' \; :. .'.1.: :';\-;:' .-;:' !!-:.;V..: l.;.^'V':; | V ' '. '/: '; : n °- 20 . ;>.^r.:*?.-.'-"'^^.;". :' ?.':' .'V^V ; ''; ''!. ; - ':' ':.'.:' V'':- -' :-' : t i ' ' ''>' '^":: ::- : " ; .^-: -'- '-.'' ''."' ' : V;--ji: ; ^f' '!. !.'' . i-'.'.' ' '' ';! 'i'JJ.:_:J-' -'-'^' j!"ir^ > T^ " ' " ".- ' ;.' O vJ v/ vJ T T . .' ; : '.;' : " "__^>JL ^naijijp ""^inC^^SS^^?^^^!Sl&^^^^':^^!^''^^Si1'^-^^Sj^:--^ ~'^^^_'c^'i L'~ r^i^ ^.'^"'^y^ ; '- -. :' ' -...- . V- ::" ' .'.= _: i.H!«^^A)*f!^J^^^ " .- ..' ' Figure 43A. Section of an acoustical profile along line 17, figure 2B, ; ; : "?_* / p ; voshowing the gentler gradient and deep seat^seated -mi-aMnnalrotational cr-ra^TTt-irgravity faultingfanlt-T-na ;v?: 1 ;.'.ll-S;-:1 =-x;: ; -.'; that is characteristic of the continentalital slope in the central sector ;: : ".;.; : '. of the continental terrace off south Texas['exas. \ \ '^'^ ": ^r^SS£gy*-:SV;.7;Mr;v^ s^#N%^^ 'tt-f-r-'^i'^-fcrxw''?*?^^^ Figure 43C. Section of an acoustical profile downslope __ £2 figure 43B showing large domal structure on the lower part of the continental .'slope caused by diapirism. MO. 20 -+3000 ft* 'r EDGE OF ; CONTINETAL SHELF . ':, ; i : >::.-- " "/ :":: r>:; '.-: >;"": ' : --.' ' '^ ^ ''^ '' ^ ^^^^^^^ :'-., ,..' i-T-'J : '. . .. ' ' ' . '--"'- . ' : -: J ' .- " .-'' . " ''. '': . -?',"''? f i <\- ' :' . . ' : ~ '.' .. .";..-.' -^ ' ""- ..-'. . :"' ,::': .-:..-..-- ^ ,. :..i.-.j.-.:.-:-:- ,.'-:""::- : -' - .-:-: ' .:-.-v!. .- ./:,. - ,. ;-'.^- ; r '.': '^';"^--,^^^i,^,:^ :; . . ';^-'f/, .:"* ' ,:' :- :-.. .' " -. -. -v.;--r.'.'" !'' '1?./" "-' ;-:'. . t . .] -- .. ; ..,:_. .-. ;_.. . ; - ;: ;,.;;.:;.: 2;.;-: ::/'; ::;-' ::',! . '..= ' .. -- ; ' Figure 44A. Section of an acoustical profile along line 39, figure 2B, showing - .; slumped and deformed older deltaic sediments beneath younger prograded sediments i and faulting associated with slumping on the continental slope (northeastern ,-\ sector of continental terrace off south Texas). ';:'^;v1:v-;:;^ / - : .-. '. '-'^i '.'; -v..^..} k'i-.'^;^;'^' .-." ' ",.:.. ". ', ' ' -^ ;' '..';'; '. " ,' :; Vv^'^"7 : '^' ;:'^'"''--' "-"!' ':-^.' ":": ' ?;:. ' -. ;-. . ' :.^^;::- '.^--'^ :-Ai' i ' ...... _.- '.. - . '.' « ' VYO. ..'..-, , V..M t.\\.. ' ".- ' >V«V.- ^'.^-. ;:-^v^ ^ ^.'1 v-;:r:^.;> sl'-^ ^r^^}SSt.^-L^^^-^ju.i-:;:r'- ^w:?..;^/:--!-":: . : :'t ,,;c. ,. -r/-..,.-- »-.!.; -.- -. <- ..-.-..'-r. -. ' ;. - r/; .; - .-.; ' ..'.. -.;-. rr y/ .- , .-,..- - -. % -.- ;..--.:-. (- -.. . , . ;.'.' SI.-A\-:.' r.-j'.- ",--« '? " ."--. ^K^'^"^.'? ;: ^^ : V?:^-'!H^ ""' '^: (4) Intensely slumped and chaotic sediments forming a relatively thin skin above older sediments that were subjected to earlier deep-seated gravity faulting rather than large scale slumping (figs. 44C, 44D, and 44E). 4. Summary: a. The edge of the continental shelf has prograded younger deltaic sediments overlying older slumped sediments of chaotic structure in the southern and northeastern parts and an active anticline parallel to the edge in the central part. Faulting has been extensive over the seaward half of the shelf. b. The continental slope is a complex of deformed sediments of various types. (Figure 45 is a map showing the areal rela tionship of the various types of deformed sediments.) c. Primary factors leading to instability: 0-1 Increase in the topographic gradient at the edge of 83 - ....'. . ; no. 154 ...... '. . . '.-.. . . .:.-.....'-.-.-- , , r, f ._,, -- - :, . no. \d(J|?n *3000 ft.» ^ ~ ^~ ' , ' ' . ;| = ~^- .;,. ; . ; ;;;; . - '...;..::."^^i^.--'-r:!rr-V r --;/'*' :l:'"- : ''. -." '" '.^';:;:^ ; nv? ;'O :V r ..''\ic ' : '' : -.;' "/ ;-:^^--'v :="J^&| : -.. i. .-.. ' . ^~T~- .! -': : -.' '. ,: - ; .- .. ., "".. - l r " " -' '* ' ' ' - - . . . ' -.-.-: ... - ^--M^sS^^W ' :.:^i)4"-:-::^ V' <4 :)''*^::-:/..»I./':!-.--? ' <' '*.'' "' . ^ '- ;. ;>: ; * '-. '':. ' ;; .f- '~"\ SEA FLOOR ^*^" ' ' ' : ' ' ^*J$ ''' ' ^t^f^^^'^^^^^^M^(' iM i Kr*-4&& >.. ^ :: :,, : :'=>'^ \I A /A74J75 ^-^ *-' . : - :; iri =^ 23 ?/i|v:'. ^';-',r 154 » .; '!a FiSure ^^C. Section of an acoustical profile downslope and con- __j^ tinued from figure 44B showing characteristic hummocky topography ^ above slumps and slides. The chaotic shallower sediments overlie ..- y. older gravity faulted sediments. __ no. i ^;;r^^^%^^;.;-3-;\;s^kJ/A ': ' -i'r Xrr.v-/ i1V-;Jv.--V:,v.':-| ; <; ^. .',. : ? * ::;? ^^i''A;y^:;'V..asV;:..:.;. »: : ;-' . -; ': J -. :" ! ! ' :' :- '.. 'i ": . ; %£.&i :.' . : -..; ::&&£-&&te%& .- f-:-: : . . ** .-. :m .; . .'- ; ..- * " ' , . : :i ' : . H^,v- ::l>vt:j;^^ vr-T-: :.:^-?^-^. >.';! ri-" .:^'?-":*: /-.; : <> -.;^ . " . . ^: '£: *' r.05 &rt v;>fv; S>^ ^ '.' >=>' ' ;.:.-';> . .w-; r-r-c \.f\f a ia ^ W: .->i'.sr^ * m' ». *^-IM . u^'*-.^ ";':-';-p '±~ .- - : ' %-; >:- < :':. -:" ?> :i,-r,V:: ^i-y '^ ^:.^?; :,|,: b:i^?; ? $&: '..-,.. ,- ;v->^v-:i;-- r: .>< mm 5>5^.^;5^? ^: :>.-; mK.i. -. ' --r>.V -T- :^;,;^_^ r^-^^^i-.'.--. i.iV:?^i't»-^ , : ' .- '^ >:^-; . T- -.'.\ :, ^:'^:-; v>:' ^«St s.-.."-: ^.g'-.-t-.V.! *?:&-, <^.*i:. .*... m -'-.' %-, o ^.-?:/v:;ov, ^* -JC,.;«i%M W.--^^ ;.V.i«Avv. .' iS||||^ "" ' ;"V; ' -:-'T ^^ : ^;;^ V \ f ..^\ .-;,!,..^ .. -/i1*.' ;V- j . ..! 96*00' 95-00' SUMMARY OF SHALLOW GEO LOGIC FEATURES,SUBAREA 3 (Continental Slope) SHELF BREAK DIAPIRIC UPLIFT CHAOTIC SEISMIC FACIES NEAR THE SURFACE OBLIQUE PROGRADATIONAL FACIES EROSIONAL ESCARPMENT SEPARATION SCAR SEAWARD LIMIT OF GROWTH FAULTS Figure 45. Shallow geologic features of the continental slope caused by deformational movements within the continental terrace, Modified from Tatum, 1977, p. 18. 87 the continental shelf where it merges with the continental slope. (2) Long term and regionally high rates of deposition along the continental terrace in general which have caused isostatic adjustments and deep-seated gravity faulting. (3) More localized high rates of sedimentation caused by deltaic outbuilding to the edge of the shelf where the combination of large volumes of unconsolidated sediments deposited on the increased topographic gradient leads to intensive slumping and sliding. (4) Diapirism or the upward movement of less dense material such as salt and shale from depth. The movement of this material upward through the thick overlying sediment causes extensive deformation and the doming of the sediments above the diapiric material as it nears the surface. (5) High gas content in rapidly deposited sediments. 88 REFERENCES (A listing of both cited and supplemental reports) '^References cited Amery, George B., 1976, Structure of continental slope, northern Gulf of Mexico jLn A. H. Bouma, G. T. Moore, and J. M, Coleman, (eds*)» Beyond the shelf break: American Association of Petroleum Geologists Marine Geology Comm. Short Course, v. 2, p. H1-H16. Antoine, John W., and Bryant, W. R., 1969, Distribution of salt and salt structures in Gulf of Mexico: American Association of Petroleum Geologists Bulletin, v. 53, no. 12, p. 2543-2550. Antoine, John W., Martin, Ray G., Jr., Pyle, T. E., and Bryant, William R., 1974, Continental margins of the Gulf of Mexico in C. A. Burk, and C. L. Drake, (eds.), The geology of the continental margins: New York, Springer-Verlag, p. 683-694. Bergantino, Robert N., 1971, Submarine regional geomorphology of the Gulf of Mexico: Geological Society of America Bulletin, v. 82, no. 3, p. 741-752. *Berryhill, H. L., Jr., 1977, Integrated environmental studies, South Texas Outer Continental Shelf: approach, techniques, results in Proceedings: 1977 Offshore Technology Conference, v. 1, no. 2754, p. 239-248. *Berryhill, H. L., Jr., Shideler, G. L., Holmes, C. W., Barnes, S.S., Hill, G. W. , Martin, E. A., Bernard, B., Pyle, C. A., Roberts, K. A., Kindinger, J. L., and Wiley, G. D., 1977, Environmental studies, South Texas Outer Continental Shelf, 1976: Geology: National Technical Information Service, Springfield, VA, Pub. accession no. PB 277-337/AS, 626 p. *Curray, Joseph P., 1960, Sediments and history of Holocene transgression, continental shelf, northwest Gulf of Mexico _in Francis P. Shepard, Fred B. Phleger, and Tjeerd H. van Andel, (eds.), Recent sediments, northwest Gulf of Mexico: Tulsa, Oklahoma, American Association of Petroleum Geologists, p. 221^-266. Ewing, M., and Antoine, J. W., 1966, New seismic data concerning sediments and diapiric structures in Sigsbee Deep and continental slope, Gulf of Mexico: American Association of Petroleum Geologists Bulletin, v. 50, no. 3, pt. 1, p. 479-504. Garrison, L. E., and Martin, R. G., Jr., 1973, Geologic structures in the Gulf of Mexico basin: United States Geological Survey Professional Paper 773, 85 p. Gealy, E. L., 1955, Topography of the continental slope in northwest Gulf of Mexico: Geological Society of America Bulletin, v. 66, no. 2, p. 203-228. 89 Hardin, F. R., and Hardin, G. C., Jr., 1961, Contemporaneous normal faults of Gulf Coast and their relation to flexures: American Association of Petroleum Geologists, v. 45, no. 2, p. 238-248. *Hill, G. W., 1976, Animal-sediment relationships jLn H. L. Berryhill, Jr., (ed.), Environmental studies. South Texas Outer Continental Shelf, 1975: Geology: National Technical Information Service, Springfield, Virginia, Pub, accession no. PB251341, p. 133-187. *Hill, G. W. , 1977, Animal-sediment relationships J.n H. L. Berryhill, Jr., (ed.), Environmental studies, South Texas Outer Continental Shelf, 1976: Geology: National Technical Information Service, Springfield, Virginia, Pub. accession no. PB277-337/AS, p. 285-389. *Holmes, C. W., and Martin, E. A., 1977, Rates of sedimentation J.n H. L. Berryhill, Jr., (ed.), Environmental studies, South Texas Outer Con tinental Shelf, 1976: Geology: National Technical Information Service, Springfield, Virginia, Pub. accession no. PB277-337/AS, p. 230-246. Holland, W. C., 1970, Bathymetric maps of eastern continental margin, U.S.A., northern Gulf of Mexico: American Association of Petroleum Geologists, sheet 3 of 3. *Lehner, Peter, 1969, Salt tectonics and Pleistocene stratigraphy on conti nental slope of northern Gulf of Mexico: American Association of Petroleum Geologists Bulletin, v. 53, no. 12, p. 2431-2479. Lewis, K. B., 1974, The continental terrace: Earth Science Reviews, v. 10, p. 37-71. Martin, R. G., Jr., 1976, Geologic framework of northern and eastern conti nental margins, Gulf of Mexico ^in A. H. Bouma, G. T. Moore, and J. M. Coleman, (eds.), Beyond the shelf break: American Association of Petroleum Geologists Marine Geology Comm. Short Course, v. 2, p. A1-A28. Martin, R. G., Jr., and Bouma, A. H., 1978, Physiography of Gulf of Mexico jLn A. H. Bouma, G. T. Moore, and J. M. Coleman, (eds.), Framework, facies, and oil trapping characteristics of upper continental margin: American Association of Petroleum Geologists, Studies in Geology No. 7, Tulsa, Oklahoma. *Moore, D. G., and Curray, J. R., 1963, Structural framework of the continental terrace, northwest Gulf of Mexico: Journal of Geophysical Research, v. 68, no. 6, p. 1725-1747. Poag, C. W., 1972, Shelf edge submarine banks in the Gulf of Mexico: Paleo- ecology and biostratigraphy: Transactions, Gulf Coast Association of Geological Societies, v. 22, p. 267-281. *Pyle, C. A., and Berryhill, H. L., Jr., 1977, Late Quaternary geologic history of the south Texas continental shelf jLn H. L, Berryhill, Jr., (ed.), Environmental studies, South Texas Outer Continental Shelf, 1976: Geology: National Technical Information Service, Springfield, Virginia, Pub. accession no. PB277-337/AS, p. 390-453. 90 *Sangree, J. B., Waylett, D. C., Grazier, D. E., Amery, G. G., and Fennessy, W. J., 1976, Recognition of continental slope seismic facies offshore Texas-Louisiana ^n A. H. Bouma, G. T. Moore, and J. M. Coleman, (eds.), Beyond the shelf break: American Association of Petroleum Geologists Marine Geology Comm. Short Course, v. 2, p. F1-F54. Shelton, John W., 1968, Role of contemporaneous faulting during basinal subsidence: American Association of Petroleum Geologists Bulletin, v. 52, no. 3, p. 399-413. *Shideler, G. L., 1976, Sea floor sediments in H. L. Berryhill, Jr., (ed.), Environmental studies, South Texas Outer Continental Shelf, 1975: Geology: National Technical Information Service, Springfield, VA, Pub. accession no. PB251341, p. 51-105. *Shideler, G. L., 1977, Sea floor sediments jLn H. L. Berryhill, Jr., (ed.), Environmental studies, South Texas Outer Continental Shelf, 1976: Geology: National Technical Information Service, Springfield, VA, Pub. accession no. PB277-337/AS, p. 142-166. *Sidner, Bruce R., 1977, Late Pleistocene geologic history of the outer continental shelf and upper continental slope, northwest.Gulf of Mexico: Ph. D. Dissertation, Texas ASM University. *Sorensen, F. H., Snodgrass, L. W., Rebman, J. H., Murchison, R. R., Jones, C. R., and Martin, Ray G., Jr., 1975, Preliminary bathymetric map of the Gulf of Mexico region: United States Geological Survey open file map. *Tatum, T. E., Jr., 1977, Shallow geologic features of the upper continental slope, northwestern Gulf of Mexico: Master's Thesis, Texas A&M University. Uchupi, Elazar, and Emery, K. 0., 1968, Structure of continental margins off Gulf Coast of United States: American Association of Petroleum Geologists Bulletin, v. 52, no. 7, p. 1162-1193. Wilhelm, Oscar, and Ewing* Maurice, 1972, Geology and history of the Gulf of Mexico: Geological Society of America Bulletin, v. 83, no. 3, p. 575-600. Woodbury, H. 0., Murray, I. B., Jr., Pickford, P. J., and Akers, W. H., 1973, Pliocene and Pleistocene depocenters, outer continental shelf, Louisiana and Texas: American. Association of Petroleum Geologists Bull etin, v. 57, no. 12, p. 2428-2439. Woodbury, H. 0., Spotts, J. H., and Akers, W. H., 1976, Gulf of Mexico continental slope sediments and sedimentation jLn A. H. Bouma, G. T. Moore, and J. M. Coleman, Ceds.), Beyond the shelf break: American Association of Petroleum Geologists Marine Geology Comm. Short Course, v. 2, p. C21-C28. 91