DRAVIS, JEFFREY JAMES. "SEDIMENTOLOGY AND DIAGENESIS OF THE UPPER CRETACEOUS AUSTIN CHALK FORMATION, SOUTH TEXAS AND NORTHERN MEXICO." (1980) PhD diss., Rice University. http://hdl.handle.net/1911/15542.

FIGURE DESCRIPTIONS

FIGURE 7. 	Presumed organic coating masking individual fine grained components of the Austin Chalk. Only a few coccoliths are barely discernable. Treatment of this sample with organic solvents failed to remove this material which suggests it may be kerogen.  Scanning electron micrograph, Preuse core chip, 1103', Williamson County, Texas.   

FIGURE 8.  
A.  Chalk sample containing an intact coccosphere comprised of individual circular plates called coccoliths.  Coccospheres are extremely rare in chalks.   Scanning electron micrograph,  Turner well cutting   chip,  16,800-16,820', Louisiana.   
B.  Chalk sample containing abundant whole and fragmented coccoliths.  At least some of the very fine   debris between whole coccoliths are disaggregated   calcite crystals which originally comprised other   coccoliths.  Scanning electron micrograph, Brushy   Creek outcrop section  (chip  -38.5'), Williamson  County,  Texas.     

FIGURE 9. Euhedral, rhombic calcite cement overgrowths  (A) on coccoliths in well-cemented Austin Chalk. Often times, it is difficult to distinguish these cements from disaggregated calcite crystals of similar morphology (B) that originally comprised coccoliths. Scanning electron micrograph, Langtry outcrop section (chip -186.0'), Val Verde County, Texas.   

FIGURE 10.  
A.  Sample of Austin Chalk composed predominantly of silt-sized planktonic microfossils  (foraminifera   and calcispheres)  in a  dark,  fine-grained mud   matrix comprised mostly of micron-sized  coccoliths and calcite cement overgrowths.  Coarser   skeletal constituents  can be very abundant  locally.  Large platy skeletal fragments  in the photograph  are  those of Inoceramus,  a  pelecypod ubiquitous  to  the Austin Chalk. Thin section photomicrograph, plane-polarized light, Langtry outcrop section  (AC-53.0'), Val Verde County,  Texas.   
B.  Closer view of sample AC-53.0', Langtry section.   Calcispheres  can  be very abundant components of   chalks  as  in this sample.  Both clear (A)  and   dark  (B)  rimmed varieties are  encountered,  Thin   section photomicrograph, plane-polarized light.  

FIGURE 11. 	Probable siliceous radiolarians (A), now replaced by non-ferroan calcite cement, in Parras Basin section. sample PB2 -4.5', Mexico. If these grains were originally radiolarians, they could have provided the silica to account for scattered chert observed throughout this section. Dark jagged seam in the photograph is a tectonic stylolite. Thin section photomicrograph, plane-polarized light.   

FIGURE 12. Photograph of lighter, glauconitic, oyster-rich (mainly whole Gryphaea) sediment overlying darker, less fossiliferous chalk. Zones such as this are commonly observed in many of the Texas subsurface cores. These lighter, coarsely fossiliferous sediments may be allochthonous. Note oyster shell riddled with borings (A). Beall core slab (5657.0'), Frio County, Texas.   

FIGURE 13.  
A.  Highly fossiliferous chalk typical of many central   Texas outcrop samples. Coarse molluscan fragments   (pelecypods, oysters, inoceramids) occur in a   finer typical chalk matrix. Aragonitic skeletal   precursors were leached by fresh water and their   molds later occluded by sparry calcite cements.   Thin section photomicrograph, plane-polarized   light, Longhorn Cement Quarry (LC-4), Bexar   County, Texas.   
B.  Deeper Texas subsurface chalk sample containing   abundant fragments of aragonitic skeletal precursors (chiefly gastropods). Such anomalous   zones occur scattered in many of the cores studied.   Polished core slab, Weinert well (6719.6'), Wilson County, Texas.   
C.  Thin section photomicrograph of Weinert core (6719.6')   depicting several originally aragonitic skeletal   fragments (mostly gastropods). Aragonitic shells   were leached in the absence of fresh water as the result of rising burial temperatures. That the molds of these grains remained uncompacted before being occluded by cement implies at least a partially lithified sediment framework to retard their compaction. Stabilization of aragonite to calcite in   this manner occurred earlier in the sample's burial   history before the onset of pressure solution. This   sample contains no noticeable pressure solution   phenomena suggesting early lithification may have   impeded pressure solution effects. Plane-polarized   light.     

FIGURE 14.  
A.  Core slab from the Texas Austin Chalk subsurface trend showing  a  large rounded calcisponge infilled   with chalk sediment of different composition than   the chalk sediment in which the calcisponge now   rests.  Rounded  form and anomalous sediment infill   suggest this fragment is allochthonous and probably   was  derived from  a  shallower water environment where   calcisponges would be expected  to be more  common   (Heckel, 1972).  Preuse core  slab  (1183.3'), Williamson County,  Texas.   
B.  Thin section photomicrograph of  a  calcisponge wall   depicting characteristic network of calcareous   spicules.  Alamo Cement Quarry (ACC-16), Bexar County,   Texas, plane-polarized light.     

FIGURE 15.  
A.  Large macroboring in an  aragonitic skeletal precursor  (clam)  infilled with ovoid  to ellipsoidal   peloids.  The original. shell wall was  leached by   fresh water and its mold,  as  well  as  interparticle   porosity between peloids, cemented by sparry calcite.   Thin section photomicrograph, Longhorn Cement Quarry,   Bexar County, Texas, plane-polarized light.  
B.  Close-up of peloids in the bored shell wall.  These   peloids most  likely were  fecal pellets.  Protection   afforded by the shell wall preserved them from   being mashed during burial.  Thin section photomicro-graph, Longhorn Cement. Quarry, Bexar County, Texas,   plane-polarized light.     

FIGURE 16. 	Disoriented intraclast from the Austin Chalk subsurface trend. Note preservation of some laminae in the intra­clast. Winterbotham core slab (6263.4'), Zavala County, Texas.    

FIGURE 17
A. Highly bored lithoclasts from the Austin Chalk subsurface trend. These rounded clasts have compositions and colors distinctly different from surrounding sediment in which they now lie, implying they have undergone transportation. These bored lithoclasts probably are submarine hardground fragments. Note that some lithoclasts have interpenetrated in response to pressure solution. Hurts core slab (4488.5'), Gonzales County, Texas. 
B. Dark; highly bored lithoclasts that probably represent remnants of submarine chalk hardgrounds. Their regular scalloped exteriors of these clasts indicate they underwent little, if any, transportation. Pholadid clams, Lithophaga and sponges were the major bioeroders of these fragments. Blumberg core slab (3270.0'), Guadalupe County, Texas.

FIGURE 18. 	Extremely glauconitic chalk from the Texas Austin Chalk subsurface trend containing coarse oyster and inoceramid debris. These glauconitic chalk zones usually range in thickness from tenths of a foot up to a foot and are a common occurrence in many of the subsurface cores.  See  FIGURE 12 for an example of similar lithology  elsewhere in the subsurface trend. Hurts core slab (4553.0'), Gonzales County, Texas.   

FIGURE 19. 	Heavily acid-etched, polished thin section of massively silicified chalk from Mamulique Pass, Mexico. Smoothed, unetched surfaces represent siliceous material as con­firmed by Energy Dispersive X-Ray analysis. Outlines of dissolved rhombic calcite crystals are obvious. Scanning electron micrograph, Mamulique Pass outcrop section (MPS -34.5'), Mexico.   

FIGURE 20.  
A.  Rhythmically bedded Austin Chalk  near Langtry,   Texas.  Thicker (1-3 foot), lighter-colored chalk   beds  are  separated by thin shales  on  the order of a   few tenths of  a  foot thick.  Note channels cutting   into some of the chalk beds.  Chalk deposition here   occurred below wave base.   
B.  Extremely well-developed rhythmic bedding at   Mamulique Pass, Mexico.  Thicker shale beds, compared   to those at Langtry, Texas,  indicate closer proximity to western terrigenous  source  areas.  These   sediments  were  deposited  below wave base and  represent the deepest water environment within the   Austin Chalk trend studied.     

FIGURE 21.  
A.  Central Texas Austin Chalk outcrop section near   San Antonio lacking well-developed rhythmic bedding.   This bedding style implies deposition above wave   base and less terrigenous input compared to more   southwestern outcrop sections.  Longhorn Cement   Quarry, Bexar County,  Texas.  Wall of quarry is   about  100 feet high.   
B.  Brushy Creek Austin Chalk outcrop section, Williamson County,  Texas.  Note the lack of distinct   shale beds and  the absence of rhythmic bedding.     

FIGURE 22. 	Typical bedding style in Texas subsurface chalks. Un­like outcropping counterparts at Langtry and in northern Mexico, subsurface chalks are characterized by lighter, relatively shale-free zones and darker, shale-rich zones. Contacts between the two generally are gradational. Varner-Wendler core slab (6043.0'), Burleson County, Texas. Scale bar to the right of the core slab is in one centi­meter increments.   

FIGURE 23.  
A.  Dark Austin Chalk containing irregular, crinkly,   light-colored "laminations" of concentrated microfossil debris.  Winterbotham core slab  (6251.0'),   Zavala County,  Texas.   
B.  Thin section photomicrograph of irregular, crinkly   "laminations" of microfossil material.  "Laminae"   are  zones  of concentrated,  cement-occluded planktonic   microfossils separated by finer darker chalk material.   Pressure solution appears capable of producing   these features which closely resemble  true deposition­  al laminations.  Mamulique Pass outcrop section   (MP2  -52.0'), Nuevo Leon,  Mexico.     

FIGURE 24.  
A.  Wispy, often bifurcating clay-rich seams  in the   Austin Chalk.  These  seams  are rarely cut by burrows.  Instead, they invariably wrap around burrows   which indicates they are not primary shale laminae   that have been contorted by compaction.  These wispy   microstylolites are the products of pervasive pressure   solution.  They are also commonly called horsetail   stylolites.  Orts  core slab  (7342.0'), Gonzales   County,  Texas.   
B.  Austin Chalk containing extremely abundant wispy   microstylolites.  Intense pressure solution,  as  he1:·e,   can  produce  a  fine nodular chalk fabric.  Orts  core   slab  (7479.2'), Gonzales County,  Texas.      

FIGURE 25. 	Relatively coarse, skeletal/glauconitic chalk flow deposit with inclined bedding, from the Austin Chalk Texas subsurface trend. This deposit is overlain by more typical, less fossiliferous chalk near the top of the core slab. Preuse core slab (1181.0'), Williamson County, Texas.  . J    

FIGURE 27.  
A.  Macroborings in a  large Inoceramus fragment  infilled   with dark chalk sediment.  These borings probably were   created by sponges.  Intensity of boring activity is   greatest in samples from the central Texas outcrops,   corresponding  to shallow water depositional environments.  Thin section photomicrograph, Alamo Cement   Quarry (ACC-16), plane-polarized light.   
B.  Numerous borings cutting across  primary fibrous   microstructure of  an  oyster fragment.  Some such   borings  are geopetally infilled.  Thin section   photomicrograph, Beall  core  (5657.0'), plane­ polarized light.       

FIGURE 28.  
A.  Thin section photomicrograph of a  planktonic   foraminifera occluded by a  mosaic of ferroan   calcite cement •. Unless syndepositionally infilled   with mud, virtually all primary intraparticle porosity becomes cemented up during the  course  of   diagenesis.  Mamulique Pass outcrop section  (MP3  -  52. 0 'A) ,  Nuevo Leon, Mexico, plane-·polarized light.   
B.  Planktonic foraminifera occluded both by coarser   blocky (rhombic)  sparry calcite (A)  and finer micritic   material  (B).  Scanning electron micrograph,  Samuels   ,core  (6975.0'), Frio County,  Texas.   
C.  Secondary moldic porosity in Austin Chalk now  oc­  cluded by sparry calcite cement  (mostly ferroan   calcite).  Primary aragonitic gastropods,  one   shown here,  were  dissolved in the subsurface in   response to rising temperatures associated with   burial and in the presence of connate waters.  Thin   section photomicrograph, Weinert  core  (6719.,6'),   Wilson County, Texas, plane-polarized light.        

FIGURE 29. 
A. 	Typical stylolitic Austin Chalk. Stylolites are very widespread, only locally abundant and restricted to lighter-colored chalks. Blumberg core slab (3214.0'), Guadalupe County, Texas.  
B. 	Nodular chalk fabric created in response to pervasive pressure solution. Distinct packages of sediment respond to pressure solution as though they were in­dividual grains. Presue core slab (1174.7'), William­son County, Texas. Reconstructed maximum burial depth for this core was probably no more than 3000 feet, demonstrating how little overburden is required to initiate intense pressure solution.  
C. 	Core slab illustrating lateral change from true stylolite into wispy microstylolitic seams. This relationship confirms the pressure solution origin of such wispy seams. Orts core slab (7455.9 1), Gonzales County, Texas.  
D. 	Tectonic stylolite produced by lateral forces assoc­iated with folding. Depositional up is toward the top of the photograph. Thin section photomicrograph, Parras Basin outcrop section (PB2 -4.5'), Nuevo Leon, Mexico, plane-polarized light.  
E. 	Thin section photomicrograph depicting interpenetra­tion of oyster fragments in response to pressure solu­tion. Varner-Wendler core (5977.0'), Burleson County, Texas, plane-polarized light.   

FIGURE 30 
A.  Thin section photomicrograph of wispy microstylolites. The lighter, irregular zones of concentrated microfossils are separated by darker, unfossiliferous, more insoluble-rich chalk. Samuels core (7122.0'), Frio County, Texas, plane-polarized light.   
B.  Close·-up view of A. Truncation of cemented foram­inifera along dark seams (A) by pressure solution is very common. Thin section photomicrograph, Samuels core (7122.0'), Frio County, Texas, plane-polarized light.   
C.  Abundant wispy microstylolites in subsurface chalk as seen. in a lightly acid-etched polished thin sec­tion, The dissolution seams stand out because they are composed of insoluble material concentrated by pressure solution (chiefly clays and pyrite). Scan­ning electron micrograph, Samuels core (7118,0'), Frio County, Texas.   
D.  Close-up of .f. demonst1.·ating dissolution of a calcite­cemented planktonic foraminifera along a wispy microstylolitic seam. Such a relationship suggests that pressure solution and cementation act concomitantly in the burial realm. Scanning electron micrograph, Samuels core (7118.0'), Frio County, Texas.     

FIGURE 31. 
A. 	Thin section photomicrograph of chalk from the flank of an anticlinal fold, While the sample is assuming a fabric reorientation in response to lateral fold stresses (depositional up is toward the top of the photograph), it still retains remnants of planktonic foraminifera cemented with ferroan calcite. These foraminifera are undergoing dissolution along vertical wispy microstylolites created by the folding. Parras Basin outcrop section (PB3 -21.3'), Nuevo Leon, Mexico, plane-polarized light.  

B. 	More intensely folded (deformed) chalk from near the fold axis. Matrix lacks preserved coccoliths and is composed of dense micrite to microspar non­ferroan calcite crystals. See  FIGURE 11 for thin section photomicrograph of similar chalk matrix. Scanning electron micrograph, Parras Basin outcrop section (PB2 -47.5'), Nuevo Leon, Mexico.  

C. 	Close-up of matrix similar to that depicted in B. Note dense, interlocked matrix calcite crystals. Scanning electron micrograph, Parras Basin outcrop section (PB2 -4.5'), Nuevo Leon, Mexico.  

D. 	Subtle fabric alignment of deformed chalk matrix in response to folding (depositional up is to the right), Probable radiolarian in the center of the photograph dissolved and was cemented with non-ferroan calcite after folding since the crystals are not aligned. This dissolution and cementation occurred in meteoric waters following folding and exposure of the section, Scan­ning electron micrograph, Parras Basin outcrop section (PB2 -4.5 1), Nuevo Leon, Mexico, lightly etched thin section.   

FIGURE 32.  
A.  Moderately altered chalk matrix composed of numerous, still well-preserved coccoliths and secondary calcite overgrowth cements. Porosity is about 10 percent. Scanning electron micrograph, Langtry outcrop section (AC -186.0'), Val Verde County, Texas.  
B.  Close-up of A illustrating the abundance of calcite overgrowth cements on remaining coccoliths.   
C.  Highly altered chalk matrix from central Texas outcrop trend. Coccoliths are rare and scattered. Most of   the material appears to be calcite overgrowth cements. This sample contained anomalous amounts of primary aragonite material and underwent significant fresh water diagenesis. Measured porosity is 17.6 percent. Scanning electron micrograph, Alamo Cement Quarry out­crop section (A.CC -17), Bexar County, Texas.   
D.  Close-up of C illustrating overgrowth cements and porosity. Many crystals are smoothed, rounded or pitted and may have been undergoing fresh water dis­solution.     

FIGURE 33. 
A. 	Relatively unaltered chalk matrix from the shallow Austin Chalk subsurface trend in Texas. Coccoliths are well-preserved, overgrowth cements are not abun­dant and porosity is relatively high (19.9%). Scanning electron micrograph, Preuse core chip (1103.0'), Williamson County, Texas.  
B, 	More deeply buried (intermediate depth) chalk matrix showing progressive loss of coccoliths and increased amounts of calcite overgrowth cements. Note size of overgrowth cement crystals compared to those in  FIGURE 33e. Scanning electron micrograph, Blumberg core chip (3225.0'), Guadalupe County, Texas.  
C. 	More deeply buried chalk containing abundant platy clay flakes, Some preserved coccoliths still remain, Porosity is low (2.6%) because of the presence of clays. Scanning electron micrograph, Orts core chip (7253.5'), Gonzales County, Texas.  
D. 	Deeply buried chalk with completely obliterated de­positional fabric. Porosity is 3.9%. Coccoliths are extremely rare., Scanning electron micrograph, Orts core chip (7490.0'), Gonzales County, Texas.  
E. 	Close-up of D. Note tightly interlocking overgrowth cements. Crystal size of these cements is distinctly smaller than those in  FIGURE 33b and indicates the overgrowth cements precipitated in smaller pores available at greater burial depths,  
F. 	Scanning electron micrograph of deeply buried, pre­sumed overpressured chalk from Louisia1.1a. Note the presence of well-preserved coccoliths. Measured porosity is 22%. Shell Turner well cutting chip (16,800-16,820'), St. Landry Parish, Louisiana.  

FIGURE 34
A.  Highly altered chalk matrix from Mexico.  Cocco­  liths are poorly preserved and the matrix is   well cemented by calcite cement  overgrowths.   Measured porosity is 1.2%.  Mamulique Pass outcrop   section (MP2  -79.0'), Nuevo Leon, Mexico, scanning   electron micrograph.   
B.  Extremely dense chalk fabric consisting of overgrowth   cements and  rare,  poorly preserved coccoliths.   Measured porosity is 0.12%.  Mamulique Pass outcrop   section (MPS  -23.1'), Nuevo Leon, Mexico, scanning   electron micrograph.     

FIGURE 35.  
A.  Light-colored Austin Chalk cut by several very thin,   vertical fractures.  In this case,  the fractures  are   healed by ferroan calcite cement.  Fractures nearly   always  are vertical and restricted to light-colored,   apparently shale-poor chalks.  Varner-Wendler  core   slab  (5954.0'), Burleson County,  Texas.  For scale,   the  core slab is 5 centimeters across.   
B.  Thin section photomicrograph of vertical healed   fracture which cuts  a  previously cemented planktonic foraminiferan.  Both the foraminiferan and fracture   are cemented by ferroan calcite.  Fractures are  late   diagenetic features,  Varner-Wendler  core  (6040.0'),   Burleson County, Texas, plane-polarized light.