GLY 3150 lecture notes by Dr. P.F. Ciesielski

GULF COASTAL PLAIN & MARGIN

I. Gulf of Mexico passive margin basin: Early History

• began to form in late Paleozoic
• may have originated as back-arc basin associated with continental collisions.
• Jurassic rifting, local structural warping, faulting (more about this in class), and localized volcanism.
• Triassic?-Jurassic, substantial evaporate formation. First deposits formed in young Gulf of Mexico.
• Jurassic -major subsidence and beginning of normal marine sedimentation of thick clastic and carbonates
• gravity tectonics, salt and shale diapirs
• early history of tectonic dams and coral algal reefs fringing most of gulf providing sediment dams
• increasing importance of diapirs as sediment dams
• by mid-Cretaceous reefs surrounded most of gulf
1. reefs buried except off Florida and Yucatan Peninsulas
2. after mid-Cretaceous sediment overflowed reef dams and built up continental rise
3. reefs buried except off Florida and Yucatan Peninsulas
   

II. Mesozoic Sedimentation

A. Jurassic

• basal Jurassic salt (extent to be illustrated in class)
1. Eagle Hills formation
2. also known as Louann Salt up to 4,500' thick
3. overlies upper Paleozoic in Ark.
4. salt grades into anhydrite up-dip and red shales
• late Jurassic transgression
1. major deposition of lime mud (to 2000')
2. consolidated into brown, chalky limestone Smackover Fm
3. the upper oolitic member a major reservoir rock
• end Jurassic regression
• sands, muds, red beds

B. Cretaceous

• landward unconformity between upper Jurassic and lower Cretaceous, elsewhere conformable
• most of Gulf Coastal Plain above sea-level in early Cretaceous, red shales and sandstones
• slow and halting transgression early Cretaceous
• complex interfingering of sandstones and limy marine facies
• dominance of marine carbonates down-dip
• dominance of red shales and sandstones up-dip
  
• mid-Cretaceous regression
• major late Cretaceous transgression as far north as Illinois, chalks and glauconite (check for evidence on geologic maps and cross-sections)
• some uplift (late Cretaceous) of Munroe Dome of La. & Jackson, Miss.
• in some of Mississippi - reefs as gas reservoirs
  
  

C. Gulf Coastal Plain- Cenozoic
( You are not responsible for formation names cited below)

• Lithologies: sands, marls, clays, and shales
• Striking facies changes down-dip and along strike
• Zones of maximum thickening and steepening lie farther inland with progressive age
• Recessional deposits dominated deltaic deposits
• subsidence more rapid during recessional phases, much thicker than transgressional phases.

First Transgression:

• Paleocene-Midway Group
1. extends to northern edge of the coastal plain, may have extended as far north as N. Dakota.
2. early Paleocene high stand "TP1" (What the heck is TP1?)
   

Paleocene Regressive Phase:

• Wilcox Sandstones -very important oil reservoirs, up to 5000 feet thick (Here we go with sands, so what?.. Think about it)

Three Eocene Cycles:

• Two constitute the Claiborne Group
• Two regressive sands (More sands):
1. Sparta and
2. Yegua continental sediments
• Transgressive Phases:
1. Crockett Shale
2. and Cane River Shale
• Final Eocene Transgression: Jackson Group
  

Oligocene:

• Mid-Oligocene major regression (Mega-regression and global marginal sands, what is it with this sand stuff?)
• major seaward progradation of deltaic deposits
• Frio Sand.
  

Neogene:

• near La coast ~12,000 m thick!!
• estimated coastal thickness 7,500-9,000 m thick
• total Mesozoic and Cenozoic thicknesses 12 to 15 km.
• Major drainage by Sabine, Trinity, Brazos, Colorado, Rio Grande, Mississippi, Tombigbee and Chattahoochee Rivers. Most modern rivers established by Miocene delivering vast amounts of sand and clay to he Gulf.
• cause for major seaward expansion into gulf during Neogene: glacial low stands!
• axis of sedimentation shifts seaward throughout the Cenozoic building the wide coastal plain (will examine maps in class)
  

D. Salt Domes and Hydrocarbons

Formation of low density salt and mud diapirs

• require substantial burial, 1200-1500 m, before sediment density surpasses salt
• structural growth by lateral salt flowage which results from:
• stresses exerted by sediment-loading up-dip and may have formed slope salt domes

Oil Accumulations:

• Eocene Yequa and Oligocene Frio sands where they wedge out up- dip
• Upper Cretaceous Woodbine Sandstone (east Texas Fields) which is sealed by the Austin Chalk
• Salt domes: oil trapped in porous beds domed and faulted by rising salt plug
1. in porous cap rock
2. along flanks where beds upturned