Sedimentary structures and environments

SWHS Geology

Horizontal Strata

Relatively flat-lying beds (>1cm) or laminations (<1cm)

Occur where sediment settle from a standing body of water or air

Laminations of Boeing Creek:

Raindrop impressionsTiny craters formed by raindrops as they impact bedding plane surfaces

Occur on muddy land surfaces

Graded beddingLayer that contains different size grains arranged from largest at the bottom to smallest at top.

Occurs where a turbulent body of sediment-rich water suddenly loses energy - flood, tidal wave, turbidity current.

Graded bedding in action!

A “turbidity current” is the

result of an undersea landslide!

Graded beds

Cross-beddingInclined beds or laminations

Occurs wherever there are wind or water currents. Direction of flow can be determined by orientation of cross-beds.

Cross bedding animations

#1: Cross beds from waveform (could be sand dune, waves on beach, etc) propagation

#2: Cross beds from waveform propagation and change in height

Cross bedded sandstone in Zion National Park

Current ripple marks

Asymmetric ripple marks - the steep slope faces down current and the gentle slope faces up current.

Occurs anywhere wind or water travels in one direction for some source of time - rivers, deserts, ocean currents

Current Ripple Marks in Discovery Park

Which way did current flow?

Oscillation ripple marks

Symmetrical ripple marks

Occur in a body of water where gentle waves barely touch bottom or weak currents move back and forth.

Ripple marks (symmetric)

MudcracksPolygonal patterns of cracks that develop in mud as it dries

Occur in muddy environments that are wet sometimes and dry at other times, like tidal mudflats or land surfaces exposed to rain

As they appear now...

...and in the rock record!

FlutesU or V shaped scrapes in mud or sand that were scoured by currents. The opening of a V or U points downstream.

Occurs wherever wind or water scours mud/sand from a surface. Strong currents are required!

Fossil plant roots/animal burrowsRoot-shaped fossils that narrow away from the main branch and tunnels or tubes that cut across layers.

Occurs wherever burrowing animals or plants live!

Fossil animal tracks or trails.Footprints or grooves left on bedding surfaces by animals.

Occur wherever animals live - may be diagnostic of kinds of animals that live in specific environments.

Fossil tracks of a Climactichnites (a 500 my old track from an unknown animal!)

What’s up with Boeing Creek?

?

Sedimentary Structures on Mars

"From the size of gravels it carried, we can interpret the water was moving about 3 feet per second, with a depth somewhere between ankle and hip deep," said Curiosity science co-investigator William Dietrich of the University of California, Berkeley. "Plenty of papers have been written about channels on Mars with many different hypotheses about the flows in them. This is the first time we're actually seeing water-transported gravel on Mars. This is a transition from speculation about the size of streambed material to direct observation of it." November 2012 - JPL