Geologic Unit Descriptions

Map Symbol

Age

Description

Qa

Quaternary

Quaternary alluvium, undivided; Clay, silt, sand, gravel, and some caliche.  Gravel includes sedimentary and igneous rock clasts.

Qal

Holocene

Clay, silt, sand (mostly quartz), gravel, and organic matter.  Gravel along Rio Grande consists of Cretaceous and Tertiary sedimentary and igneous rock clasts; also includes sidestream alluvial gravels consisting of Tertiary rock clasts and chert derived from the Uvalde gravel.

Qam

Holocene

Muddy floodplain alluvium; Floodplain deposits of the lower Rio Grande; mostly mud.

Qas

Holocene

Silty and sandy floodplain alluvium; Floodplain deposits of the lower Rio Grande; mostly silt and sand.

Qtf

Holocene

Tidal flat deposits; Sand, silt, mud, and shell.

Qbi

Holocene

Barrier island deposits; Mostly fine-grained sand, but also includes silt, sand, and shell; unit becomes more muddy landward.  Deposited in beach ridge, spit, tidal channel, tidal delta, washover fan, and sand dune environments.

Qsd

Holocene

Active dunes on mainland; Eolian deposits consisting of mostly sand and silt forming banner and brachan dunes elongated in northwest direction and developed on stablilized dune (Qds) surfaces.

Qcd

Holocene

Clay dune and clay-sand dune deposits; Eolian deposits consisting of calcareous clay, silt, and sand; mostly clay and silt in coastal areas, and clay, silt, and sand inland.  Dunes are exposed mostly on downwind side of intermittenly wet basins.

Qds

Holocene

Stabilized sand dunes; Eolian deposits consisting of mostly sand with relict eolian grain.  Exposed in blowouts and small depressions.

Qs

Holocene

Sand sheet deposits; Eolian deposits consisting mostly of sand with little or no relict grain.

Qcg

Quaternary

Quaternary conglomerate; clasts are mostly limestone with some sandstone; minor calcium carbonate cement.

Qeo

Quaternary

Eolian deposits consisting of a mixture of quartz sand and shell fragments deposited mostly in sand dunes along the coast line.  Unit also includes some clay dunes.

Qla

Quaternary

Coastal Lacustrine deposits; Mixture of silt, mud, and organic matter exposed along the coast in the Mexican Laguna Madre area.  Unit contains shell fragments of oysters, bivalves, and gastropods; also some oolites, dessication cracks, and halite crystals.  Halite is mined locally.

Qli

Quaternary

Littoral deposits; Mixture of sand and shell fragments (mainly bivalves and gastropods).  Unit exposed along coastline in beach and barrier bar depositional environments and is the principal sediment source for eolian deposits.

Qt

Pleistocene

Terrace deposits; Rio Grande river terraces consisting of gravel, sand, silt, and clay; exposed on north side of Rio Grande from Falcon Reservoir to Los Ebanos.

Qb

Pleistocene

Beaumont Formation, undivided; Clay, silt, sand , and gravel; stream channel, point bar, natural levee, and backswamp deposits.

Qbf

Pleistocene

Beaumont Formation, floodplain deposits; Floodplain deposits of the Beaumont Formation consisting of mud veneer over meanderbelt sand.  Little or no relict grain; grass covered.

Qbm

Pleistocene

Beaumont Formation, clay and mud; Mostly mud and clay of Beaumont Formation deposited in meanderbelt, levee, crevasse splay, and distributary channels.

Ql

Pleistocene

Lissie Formation; Clay, silt, sand, gravel, and caliche.

QTu

Pleistocene and Pliocene

Uvalde Gravel; Gravel consisting of well-rounded chert clasts.  About 6 m thick.

Tc

Pliocene

Caliche; Massive and dense caliche and some sand and gravel.  Unit generally lies disconformably on Tertiary bedrock units.

Tpcg

Pliocene

Pliocene conglomerate; Massive conglomerate beds and some lenses of fine-grained sandstone. Conglomerate has clasts of chert, intrustive rocks, and limestone; matrix composed of sand and silt; cemented with carbonate.

Tgc

Pliocene

Goliad Formation with caliche

Tg

Pliocene

Goliad Formation, undivided; Clay, sand, sandstone, marl, caliche, limestone, and conglomerate.

Tfo

Miocene

Flemming Formation and Oakville Sandstone, undivided; Crossbedded sandstone and sandy shale; partly oolitic; some beds contain abundant oysters and bivalves. Unit lies disconformably on Oligocene units, and disconformably below Pliocene units.

Tcv

Miocene and Oligocene

Catahoula, Frio, and Vicksburg Formations, undivided; Mudstone, claystone, sandstone, tuff, and clay.

Tsi

Oligocene

Intrustive rocks, syenite; Nepheline syenite; essential minerals include orthoclase and oligoclase; accessory minerals include quartz, apatite, and zircon.

Tj

Eocene

Jackson Group; Mostly sandstone and lesser amounts of clay. Unit is calcareous and contains fossil wood fragments, volcanic ash beds, and limestone concretions.

Ty

Eocene

Yegua Formation, Claiborne Group; Mostly clay and lesser amounts of sandstone. Some beds are lignitic and bentonitic; some fossil wood. Sandstone is partly glauconitic and ferruginous.

Tjw

Eocene

Jackson Group, Claiborne Group, and Wilcox Group, undivided; Alternating beds of shale and sandstone; sandstone is ferruginous and glauconitic; carbonaceous sandy shale beds contain gastropods and pelecypods; unit contains some volcanic material such as detrital mica.

Tl

Eocene

Laredo Formation, Claiborne Group; Sandstone and clay. Abundant marine fossils and common limestone concretions.

Tmid

Paleocene

Midway Group, undivided; Claystone, sandstone, and shale;  some bentonite and ferruginous and calcareous concretions.  Sandstone is locally glauconitic.  Unit has cone-in-cone structures, trace fossils, and contains some oysters and turritella.  Unit from 0 to 1,250 m thick.

Km

Upper Cretaceous

Mendez Shale; Fissle calcareous shale; hematite nodules; locally includes some intraclastic micrite and calcareous sandstone beds in lenses.  Maximum thickness greater than 1,300 m.

Kae

Upper Cretaceous

Austin and Eagle Ford Formations, undivided; Interbedded argillaceous limestone and calcareous shale; nodules and layers of iron oxide; some horizons of bentonite and sandy limestone; Austin contains some chert layers; basal and upper contacts of unit are transitional.

Kwt

Lower Cretaceous

Washita Group, Kiamichi Formation, Aurora Limestone, La Pena, Cupido, and Taraises Formations, undivided; Washita, Aurora, and Cupido are mostly micrite with black chert and hematite in layers and nodules. Kiamichi, La Pena, and Taraises are mostly argillaceous limestone and calcareous, silty shale.

Jlc

Upper Jurassic

La Casita Group; Limestone, sandy clay, and calcareous shale; shale is laminated to fissile; sandstone is fine-grained and has hematite and calcareous concretions cored by ammonites and pelecypods.  Lower contact with Zuloaga Group is sharp; contact with overlying Taraises Formation is transitional. About 300 m thick.

Jz

Upper Jurassic

Zuloaga Group; Mostly limestone with scarce chert; limestone is oolitic and contains sparse mollusk fragments.  Unit also includes some interbeds of anhydrite and gypsum.  About 130 m thick.

Jm

Upper Jurassic

Mina Viejas Formaton; Anhydrite and halite with interbeds of limestone, shale, sandstone, and bentonite. Unit exposed in the core of an anticline in deformed Cretaceous and Jurassic rocks on the west edge of the map. Unit from 0 to 200 m thick.