Petrified Forest National Park

Located in Northwest Arizona Petrified Forest Declared Nat’l Monument Dec. 8, 1906 Painted Desert added Dec. 9, 1962 when whole area was made Nat’l Park. Covers 93, acres

Petrified Forest Painted Desert

Paleo-Climate Bedrock Composed of Mesozoic Sedimentary Rocks. During the Late Triassic, region was on the southwestern edge of Pangaea just north of the equator. Climate was humid and sub-tropical Large river system covered the area Chinle Formation Deposited during this time period

Chinle Formation Shinarump Member: Basal conglomerate made up of Paleozoic clastic igneous cobbles Lower/Upper Petrified Forest members = Primarily sandstone Black Forest Tuff = Shale contains petrified logs

Chinle Formation Cont’d As oceans retreat in the Late Triassic, park area becomes site of river system and flood plains depositing muds and silts of the Black Forest Tuff. Petrified forest deposited in Black Forest Tuff.

How to Petrify a Forest Large, pine like trees felled by flooding in Late Triassic Silts and muds deposited from fluvial/ lacustrine environments on top of fallen trees Volcanic activity in and around the area deposits layer of volcanic ash Layers of sediment cut off oxygen to fallen trees, slowing their decay Groundwater, rich with silica from volcanic ash seeps into the fallen trees, slowly preserving its tissue structure with silica deposits. Given time and pressure, silica becomes quartz and logs are preserved as petrified wood, colored with oxides of iron and manganese

Painted Desert Chinle Formations coloration reflect its ancient soil horizons. Red and Green layers caused by Iron and Manganese – Differences in color come from the position of the water table when the soils were formed High water table  Lack of Oxygen in Sediments  Green/Blue hues in Iron minerals Low water table  More Oxygen Available  Reddish hues from oxidized iron minerals Reflect seasonal flooding throughout Triassic

Pliocene / Quaternary Deposits Unconformity between Triassic and Pliocene leaves no geologic record until Bidahochi Formation (192 million yrs) Bidahochi Formation = more fluvial and lacustrine deposits ( silt clay, sand), and volcanic ash deposits. Today Most fluvial/ lacustrine deposits of Bidahochi Formation has been eroded away, leaving exposures of resistant volcanic basalts. Quaternary Deposits = windblown dunes and land sediments.

Volcanic Basalts 

Effects of Erosion: The look of the park. Wind and water erosion over millions of years, in addition to the significant uplift after the Triassic, has sculpted the hills, mesas and buttes that have come to characterize the park.

Sources fo/ fo/ parks.com/petrified/geology.html parks.com/petrified/geology.html etrified_forest/geology.html etrified_forest/geology.html lfossilwood.htm lfossilwood.htm

Zion National Park by Libby Kugel

Located in the southwestern corner of Utah Established in 1909 as Mukuntuweap National Monument In 1919 the monument was expanded to become Zion National Park Will celebrate centennial in 2009 Utah’s first national park

The Grand Staircase Nine exposed formations in Zion are part of a super- sequence Where the Grand Canoyn’s story ends, Zion’s story begins The youngest formation in Zion is the oldest in Bryce Canyon – The Dakota Sandstone Formation

Depositional History Mud, silt and sand deposited Weight caused the land to sink at an even rate, maintaining the elevation, until it sank below sea level and flooded The compression and the mineral bearing sea water caused the sediments to turn to stone

Depositional History Permian – Deposited in warm, shallow seas Kaibab Limestone Formation Triassic – Moenkopi Formation – Chinle Formation Jurassic – Deposited in streams, ponds and lakes Moenave Formation Kayenta Formation – Deposited in desert Navajo Formation Temple Cap Formation – Deposited in dry, near-shore environment Carmel Formation Cretaceous – Dakota Sandstone Formation

Uplift occurred over the Colorado Plateau Rivers increased in strength and began to carve into the rocks Virgin River carved out Zion Canyon Uplift still happening today – 5.8 Earthquake in

Erosion Downcutting – Zion Narrows Canyon Widening

Arches Freestanding arches are a unique feature to the park Part of the Navajo Sandstone formation Kolob Arch

References Zion National Park: A Tweak of History and a Tweak of Geology, National Park Service, cfm cfm o_zion.htm o_zion.htm tm tm