Stratigraphy San Ysidro

Anticline and Syncline

Jemez Mountains Valle Caldera

San Ysidro

An eroded anticline and syncline make up the major geology near this little town with late Triassic to early Cretaceous sedimentary rocks exposed in the folds.

Late Triassic – Mid Jurassic UnitsThe core of the anticline contains the Painted Desert Member of the Petrified

Forest Formation (Chinle Group). A red to green siltstone and mudstone deposited on the floodplain of a large northwest flowing river system about 205 – 210 Ma.

The Middle Jurassic Todilto formation lies directly on the Chinle group in this

area.

View toward the north of the east limb of the anticline. TRcpd is the Triassic Painted Desert member of the Petrified Forest Formation (Chinle Group) and Jt is the Jurassic Todilto Formation. The mountains on the skyline are the Sierra Nacimiento. Photograph taken by Richard Kelley.

.The Todilto Formation was deposited in a salina about 159Ma during the

Jurassic ocean, and is composed of two units;

•Mesa Luciano Member – a basal laminated shale and limestone•Tonque Arroyo Member – an upper gypsum deposit

The Summerville Formation lies above the Todilto and was deposited on an arid coastal plain

•Summerville Formation – a maroon mudstone and pinkish-tan, poorly cemented sandstone.

The Morrison Formation has two units exposed in this area;

•Salt Wash Member - this is a river channel sandstone making up the basal unit of the Morrison. It has distinctive gold colored cross-bedded sandstone and red to green silt stone. The sandstone contains a good amount of concretions and form the west-dipping beds on the eastern flank in the anticline.

•Brushy Basin Member – interbedded salmon pink to a pistachio green siltstonewith thin river channel sandstone. This formation has preserved dinosaur bones and highly polished gastroliths. Radiometric dating of ash beds contained in the Brushy Basin in Utah and Colorado place the age of this member at 148 – 150 Ma ( Kowallis, 1998)

View of the east limb of the anticline, looking north toward the Sierra Nacimiento. Js is Jurassic Summerville Formation, Jms is the Salt Wash Member of the Jurassic Morrison Formation, and Jmb is the Brushy Basin Member of the Morrison Formation. Photograph by Richard Kelley

The top of the Morrison formation is composed of the Jackpile Sandstone.

•Jackpile Sandstone Member – River deposit.

East limb of the anticline exposing the Brushy Basin Member and the Jackpile Member of the Morrison Formation (Jmb and Jmj) and the basal unit of the Dakota Formation (Kd). Photograph by Richard Kelley.

Cretaceous Units

The Cretaceous units exposed here are coastal plain, shoreline and marine deposits from along the western area of the Western Interior Seaway about 93 – 125 Ma yrs ago.

•Dakota Formations - base is distinguished by the chert and quartzite pebbleconglomerate that was deposited on and irregular surface. This formation is composed of six units in this area, five of which are in continuous sequence. The top unit is separated by a tongue of the Mancos Shale.

•Encinal Canyon Sandstone – the basal unit, a thin bedded, cross-bedded, goldcolored sandstone interbedded with carbonaceous beds and black shale.

•Oak Canyon Member – a shale containing plant material fossils interbedded withthin bioturbated sandstone beds. The sandstones are cross-bedded locally and intensely burrowed.

•Cubero Sandstone – gold colored, bioturbated sandstone•Clay Mesa Member – black marine shale•Paguate Sandstone – orange – yellow sandstone that has been bioturbated, and

contains ripple marks.-A thin tongue of the Mancos Shale overlays the Paguate sandstone here

•Twowells Member - Sandstone

The Jemez MountainsThe Jemez mountains surround the Valle Caldera, One of the largest volcanic craters in the world.

There are some Triassic age stratagraphic units exposed on the southern end of the Jemez uplift though. - Anton Chico Member of the Moenkopi Formation. This unit is the oldest unit Exposed here and overlies disconformably, middle Permian sandstones that are relatively thin units (up to 39m) recognized as red-bed sandstones. - Agua Zarca Formation of the Chinle Group overlies disconformably the Moenkopi Formations and are a thick sequence ( up to 400m) of red-bed mudstones with a fairly thick basal unit ( up to 61m) of sandstone and extrabasinal conglomerates.

Triassic Units Chinle Group

•Agua Zarca Member sandstones are white to brown, med-coarse grained quartz arenite, typically crossbedded conglomerates. The dominant clast are Metaquartzite pebbles up to 15cm with some red jasper, and petrified wood mixed in.The Agua Zarca generally fines upward and conformably grades to the Salitral.

The Salitral Formation is comprised of mostly purple, blue and red mudstone with Calcareous concretions and the maximum thickness of the Salitral is 102m but is Usually thinner.

•The Poleo Formation sandstone and conglomerate dominated unit resemblesThe Agua Zarce Formation. However the Poleo sandstones are gray micaceousLitharenits. The Conglomerates are dominately calcrete clasts with siliceous Clast being uncommon. The Poleo maximum thickness is 40m in northern areas,And thins southward into the Jemez uplifts. The base of this unit is a Disconformity while conformably overlain by the Petrified Forest Formation.

•The Petrified Forest formation is dominated by reddish brown mudstone that is the Painted Desert Member. It also contains many thin beds of crossbeddedOr laminated micaceous litharenite and calcreteclast dominated conglomerate.The upper part of the Petrified forest formation is the correo sandsone bed of The painted desert member and is disconformably overlain by the Middle Jurassic Entrada Sandstone.

Structural Features

Cabezon Peak, an eroded volcano throat, near San Ysidro

Structural history: 75 million years ago Laramide Orogeny

Jemez Mountain Range

Jemez Mountain Range Cont.

Jemez lineament is a NE trending crustal flaw that controlled volcanism and tectonism in the Jemez Mountains and the Rio Grande rift zone

Volcanism did not begin until after the Sante Fe rift The volcanic rocks lie uncomformably on top of

igneous, metamorphic, and sedimentary rocks Ages range from Precambrian through late

Tertiary. Mesozoic sedimentary rocks form the eastward-

dipping backslope of the Sierra Nacimient The Valles Caldera truncated an older volcanic

flow

Jemez Mountain Range Cont. The western margin of the Rio Grande depression in the

Jemez area is delineated by a zone of northeast-trending faults.

An en echelon zone is expressed by the Pajarito fault zone in the San Miguel Mts

North of Santa Clara Canyon,fault zone is a series of antithetic faults that displace the basalts along the eastern edge of Lobato Mesa.

Faults in the West generally show more normal displacement downward to the east with Right-Slip faulting

Faults in the East are antithetic (displacement is opposite to faults ) with Left slip faulting

Fun Fact: Due to such high geothermal volcanic activity, the Jemez Mountains contain several hot springs, making them a tourist Hot Spot

Jemez Mountains Cont.

Sandia Mountains

Sandia Mountains Dominated by an eastward-tilted fault block An unusual structure of the area is the Tijeras

rift belt bounding the uplift that trending northeastwards and contains a southwestern folded graben and a northeastern horst

Tijeras rift belt was formed in the Laramide Orogeny

North-east striking, subvertical faults : Tijeras, Gutierrez, San Lazarus, Los Angeles, and Lamy faults

Sandia Mountains Cont.

Tijeras fault is a Left-Lateral Slip Fault system links to north-striking

Picuris-Pecos fault system Tijeras–Cañoncito fault system is

characterized bysubparallel faults, Horsts and grabens are present along

much of its length.

Sandia Mountains Cont.

Albuquerque Basin The basin is part of an orogenic belt

dominated by a rift valley system To the west is the Colorado Plateau, the

Sandia fault to the east, the Nacimiento uplift, into the Jemez Mts, and La Bajada scarp to the north, and the Socorro constriction to the south.

consists of two subbasins downdropped along low-angle to listric, normal faults of opposing structural polarity seperated by a mid-basin transverse structural zone

Lithology

Mancos Shale

Deposited by an ancient inland sea during the Cretaceous time period.

Source: geosciences.fau.edu

Dakota Formation

Deposited during Early Cretaceous and earliest Late Cretaceous in response to a regional transgression.

Source: bforist.blogspot.com

Morrison Formation

Consists of sandstone and mudstone deposited in fluvial environments.

Source: en.wikipedia.org

Summerville Formation

Summerville deposition took place on a vast arid coastal plain that extended from Utah to Oklahoma (Anderson and Lucas, 1992, 1994; Lucas and Anderson, 1997).

Source: urth-picturepost.blogspot.com

Todilto Formation

Is limestone that was deposited by a saline inland sea.

Source: fs.usda.gov

Chinle Group

Deposited on widespread alluvial plains of low relief.

Source: schmunda.blogspot.com

Questions?