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Lab 10 Sedimentary structures worksheet


									Name _________________                                           Geology 103A

                            Sedimentary Structures

Objectives: In this lab we will observe examples of some of the common
sedimentary structures. Your assignment is to examine the display specimens,
find each sample listed below, and answer the questions about sedimentary
structures. When you have answered all of the questions show me your paper
and I will let you look at the answer key. Correct your paper, then go back and
look at samples that you got wrong. Before you leave you will take a short exit
quiz to test your knowledge of sedimentary structures.

I) Lamination: Beds that are less than 1 cm thick are called laminae. Laminae
occur in many different types of sedimentary rocks, and usually indicate
deposition by slow-moving currents or standing water. Examine the laminated
rocks listed below, and fill in the blanks for the two unknown rock types:

      Sample                                 Rock name
      LCB                                 laminated chert (sedimentary)
      LPR                                    ___                 ________ _
      FEL                                    ___________                  _
      MQL                                 laminated quartzite (metamorphic)
      VTL                                 laminated tuff (volcanic)      __

Note that lamination may occur in many different lithologies, and is not restricted
to shale or siltstone.

II) Cross lamination:
Sediments that contain ripples or dunes may show cross lamination. Cross
lamination is a common feature, and describes the case where ripples or dunes
erode and deposit on top of each other. This interference between different
bedforms prevents complete preservation of individual ripples or dunes. It is
common for the foreset beds to be preserved when cross lamination forms, but
other parts of the ripple may not be present.

ORM- Careful, fragile sample!!! Did the ripples that deposited this silty
    sandstone have straight crests?(Hint- look at the surface labeled “ORM”)
                        What name would we give to these ripples?
    Which direction was the current flowing (use the labels a,b,c and d to refer
    to the current direction)?                Which way is "up" on this sample
    (use the numbers to refer to the "up" direction)? _____________

TSU- Cross-beds in this sample tell which direction was "up" when the rock was
     deposited. Is the TSU label written right-side-up, or upside-down?
     (Hint: when ripples form, the foreset laminae do NOT trunctate abruptly
     against the underlying bed. Instead, the angel of the foreset laminae gently
     decreases, and approaches horizontal.

SUR- What type of current deposited these ripple marks?

QXS- The large-scale ripple marks in this sample are almost large enough to be
     called dunes. Which way was the current flowing when these ripples were
     deposited? Use the numbers to show where the current is flowing from
     ________ and toward _________. Which direction is "up" - A or B?
     (Hint: use the ripple mark outlined with black arrows to answer this
     question. Think about foreset beds migrating in a downstream direction as
     the ripple forms)

UQR- What would you call these ripples? They have symmetrical crests in cross

BRS- Look at the crests (tops) of these ripples. What name would you give to
     ripples that have crests that split and rejoin?

RSR- These ripples are assymetrical and straight-crested. What name do we
     give to ripples that are deposited by a current that flows constantly in one

III) Mixed and discontinuous internal bedding: Changes in current velocity or
sediment supply cause alternation of sandstone, siltstone and clay laminae.
Answer the following questions or observe the features that are described in
these examples of mixed and discontinuous bedding:

PLE- (Observe only) Sample PLE is an example of lenticular bedding in a
      sequence of interbedded fine-grained sandstone (light color) and siltstone
      (dark color).

MUC- This is a low rank metamorphic rock that formerly consisted of sandy beds
     (now tan-colored, and labeled with the number “2"), and silty shale (dark
     laminae labeled “1"). IF the silty beds were isolated ripples, encased in
     sandy beds, what would we name this sedimentary structure?

DCS- Look at your list of bedding terms. Is the bedding near the “DCS” label
     continuous? ________ What descriptive terms would you use to describe
     this type of bedding?

SRU- (Observe only) This sandstone contains large, flat clasts up to 5 cm long
     that are called rip-up clasts. Rip-up clasts form when a current erodes
     cohesive pieces of the nearby bottom or bank and re-deposits these clasts
     downstream. This type of bedding is sometimes called “chaotic”, but could
     also be describe as _____________, ______________, ______________.

CSH- What two lithologies are present in this sample?              ,          .
     The coarser bed (labelled "1") is sometimes referred to as a lense. What
     term would we use to describe isolated sandy ripples encased in finer-
     grained laminae composed of siltstone or shale?

CWB- What name do we use to describe alternating beds of siltstone and shale
    that occur in sub-equal proportions?                         . What two
    conditions cause this type of bedding?                 ,

SQC- Describe the change in grain size from A to B:
     If B is the “up” direction, this sedimentary structure that forms within an
     individual bed is called __________________

IV) Irregular stratification:

WDF- Careful- fragile sample!! The curvedm irregular bedding near the “WDF”
     label forms when the sediment deforms while it is still wet and plastic. It is
     called                             .

FES- Sample FES has examples of 2 sedimentary structures that form
     during de-watering or slumping of soft sediment. The features labelled "1"
     form when fluid escapes upward in soft, liquefied sediment, disrupting
     bedding or laminae. It is called                       . The feature
     labelled "2" is a                , and forms when soft sediment slumps
     gently downslope.

BSM- FRAGILE SAMPLE!! Sample BSM contains ripple marks, soft-sediment
     deformation (convolute bedding) and sole marks. Was the current that
     formed the sole marks flowing toward "a" or toward "b"?            Is this
     the top or bottom of the bed?

V) Bi-directional current indicators: Elongate sedimentary particles tend to
orient parallel or perpendicular to the current direction. When this orientation is
visible, it is possible to estimate the trend but not the absolute direction of the

PLS- The faint grain alignment in this sandstone produces a linear texture on
     bedding plane surfaces. Subtle grain orientation gives the rock a striated
     texture, and indicates that flow was either from a to b or from b to a. It is
     not possible to tell the exact current orientation. This sedimentary structure
     is called

OLM- (Observe only) This silty sandstone contains plant debris that shows the
     trend of the depositing current. Elongate plant fragments in this sample are
     oriented in two directions. Flow was along one of the marked axes,
     although it is not possible to tell which axis represents the current direction.
     Elongate clasts may either orient parallel or perpendicular to the current
     direction, and in this case they have done both!

VI) Bedding plane markings, biogenic structures:

SMP- FRAGILE SAMPLE!!! This is a Lower Pennsylvanian Stigmaria root cast
     from Jackson, Ohio. Note the root hair attachment sites on the edges of
     the root. Do root structures usually indicate a terrestrial or marine
     depositional environment? ____________________

QFS- This bedding plane structure is produced by currents, and is called
                     . The current that deposited this structure was flowing
     from                to                 . Is this the top or bottom of the
     bed?                    .

BTW- (Observe only) The tubes in this sample were formed by burrowing soft-
     bodied animals (probably worms). Note the smooth internal linings. These
     linings are often cemented by mucous when the organism is alive.

PTB- This is an escape structure called Skolithos that formed when the organism
     (probably a soft-bodied worm) was buried by a sudden influx of sand. The
     worm? tried to dig escape by digging upward and back-filling the burrow.
     Assume that the label is oriented with the top up. Did the work make it?

BTT- FRAGILE SAMPLE!! This sample shows tracks, trails and burrows on the
     bottom of a sandstone bed. Can you find the feeding traces that were
     probably formed by the legs of a trilobite? What type of ripple marks
     formed on the top of the bed? ________________, and what environment
     is this sample did this sample form in?________________

POH- This Paleozoic sandstone shows a network of burrows called
     Arthrophycus. Burrows may occur internally (within beds) or on the surface
     of beds. Which type of burrow is this? ___________________

MBS- What direction was the current flowing when it formed these flute casts?

      ____________________ to _____________________

HHC, HHS- What formed the surface markings on these rocks? Hint- think of a
     large biogenic influence

BWY- This fine-grained sandstone from the Eocene Bridger Formation
     (Wyoming) shows areas where the bedding has been disrupted and
     homogenized (see circled areas). What caused this? _______________

VII) Secondary sedimentary structures and crystal growth:

CSC- (observe only) This "cannonball" is a concretion that formed by secondary
     mineralization of the sandstone. The internal bedding is horizontal even
     thought the cemented shape is spherical. Can you see the internal

FIS- (Observe only) This classic concretion has an organic nucleus (fossil)
     surrounded by a cemented sphere. The pyrite or marcasite mineralization
     forms in a reducing environment, possibly as a result of organic decay of
     the fossil nucleus.

BTW- (0bserve only) This is a concretion. Concretions form when a secondary
     (post-depositional) mineral cements the rock. Many concretions form
     around organic nucleii and grow outward with a spherical shape. Note that
     the bedding cuts across the spherical shape.

RGR- Fragile sample!! Pliocene Camp Rice Formation, New Mexico. Some
     cements that form concretions show gravitational influence, like an icicle or
     stalactite. Is the label on the top or bottom of this sample?

PPC, DRG, PLS, (Observe only) These samples formed when a secondary
     mineral cemented the original sandy material. In each case, the crystal
     morphology of the cement is preserved in the hand specimen. DRG was
     cemented by gypsum, and is commonly called a "desert rose". PPC was
     cemented by calcite, and PLS has voids where crystals of gypsum once
     existed. The gypsum has dissolved, leaving crystal-shaped voids.

PLS-2: Cubic crystal form on the top of sample PLS-2 may indicate the growth of
     secondary crystals called "hopper crystals". What mineral might form
     these cubic crystals, then dissolve? ________________.

PPC- What is this sedimentary structure formed by bladed gypsum crystals

VIII) Mudcracks, raindrop imprints:

Observe only: large, unmarked mudcrack sample

RSC- This red siltstone has square impressions where halite crystals formed.
     Some of the more circular impressions may be raindrop imprints.

SMS- The intersecting ridges are mudcracks. Mudcracks are preserved when
     sandy material is swept into open cracks. This is common where
     expandable clays (smectites) are present, and frequently occurs where the
     bedrock, soil or sediment have a volcanic influence. Is this surface the
     bottom or top of the bed? _________________

MCB- When mudcracks form, they usually form in finer grained silt and mud.
     What material has infiltrated into the mudcracks, and preserved them?

IX) "Fake" sedimentary structures:

Observe only- Look at the large tray of unmarked samples. Many of these are
fossils or later mineralization, and are not related to bedding or sedimentary
structures. Would you be fooled by these features?

X) Concretions, Concretions, Concretions:

Observe only- Look at the two large trays of unmarked samples. Concretions
may have a wide variety of shapes and sizes.


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