"Timothy Demko" wrote
Cross beds I take it simply means that the layers are not always
parallel--they diverge and converge sometimes? If so, how about this
photo
of the effect of "volcanic bombs":
http://www.geo.hunter.cuny.edu/~cwang/impacts.jpg Wouldn't the layers in
that photo be a simple air fall?

The presence of cross beds means something more than the beds just not
being parallel. The beds in the link you provided would be called
deformed beds rather than cross beds. Cross beds formed by the migration
of bedforms (ripples, dunes, etc.) and are a primary featu they form
during, and are linked to, deposition. These bedforms may be subaerial
(eolian) or subaqueous. In deformed beds, the deformation event, in your
example the impact of a falling volcanic bomb, happens after the
deposition of the beds.

Thanks for clarifying that. I managed to find this short USGS video which
helped me to visualize it:

http://tinyurl.com/ysoda


"Secondly, they could be some kind of precipitate * perhaps from water.
In
additional findings, there are some minerals that have been identified
in
the infrared that would suggest the presence of water since they form in
water. In particular, if this is a volcanic terrain that we are looking
at,
the presence of these minerals might suggest this was a hydrothermal
area."

What--lapilli are fading back in and the other two theories (magma &
concretions) are gone, to be replaced with "some kind of precipitate"?

No, I think he is talking about concretions or nodules. The confusion is
in that most people are assuming that the precipitation was happening in
a water column or on a lake/sea bed. I think what Greeley means here is
that he thinks they formed by prepitation from water in the subsurface
(pore waters), and he is leaning towards a hydrothermal origin for the
water, and maybe mineralization too.

OK, my misunderstanding as a google-ogist rather than a geologist. When I
hear precipitate I think of something falling, like through said column of
water.


snip

AND ALL THIS IS ADDING UP TO A QUESTION ABOUT WHETHER THE PLACE WHERE
OPPORTUNITY CAME DOWN ORIGINALLY MIGHT HAVE BEEN HOT MAGMA COMING UP TO
THE
SURFACE IN SOME KIND OF VOLCANIC ACTIVITY? OR AT LEAST HOT SPRINGS?

"Yes. The evidence is sort of pointing towards the presence of a hot
environment * hot for some reason and that's under debate right now.
Volcanic activity, magma close to the surface, is one possibility."

...

THE BIGGEST SURPRISE RIGHT NOW HAS BEEN THE HIGH AMOUNT OF SULFUR IN THE
BEDROCK?

"That was a surprise, yes. The measurements that have been made there
suggest there is a very high percentage of sulfur present in the
material
and that there are these spherules that have been seen and reported
earlier,
those are still very curious features."

"Such a high percentage of sulfur (in the bedrock) is unusual and this
is
what leads some folks to consider the volcanic origin (at the Martian
bedrock), or at least volcanic processes to be involved. There are some
minerals that have been identified in the infrared that would suggest
the
presence of water since they form in water. In particular, if this is a
volcanic terrain that we are looking at, the presence of these minerals
might suggest this was a hydrothermal area and sulfur is a component of
such
environments."

If you look through the older Mars literature, the origin of soil
minerals (including sulfur minerals) seen at the Viking and Pathfinder
sites if often related to something called "palagonitization". This a
garbage can term for various geochemical processes involved in the
weathering and divitrification of basaltic glass (palagonite is an
amorphous to poorly crystalline, almost clay-like substance).
Palagonitization does involve hydration and it is common in hydrothermal
areas where hot thermal waters are circulating through volcanic and
volcaniclastic rocks.

If we read between lines then, it would seem that the MER project
scientists are leaning towards a palagonitized, volcaniclastic
(re-sedimented tuff/ash) origin for the bedrock, and a subsequent
diagenetic/hydrothermal alteration origin for the sulfur anomaly and
hematite-rich spherules.

OK, had to look that up:

diagenesis: The physical, chemical or biological alteration of sediments
into sedimentary rock at relatively low temperatures and pressures that can
result in changes to the rock's original mineralogy and texture. After
deposition, sediments are compacted as they are buried beneath successive
layers of sediment and cemented by minerals that precipitate from solution.
Grains of sediment, rock fragments and fossils can be replaced by other
minerals during diagenesis.

Note that these processes may all have been related to the same event of
series of events. Maybe something like: 1) rise of magma through the
mantle and crust of Mars; 2) eruption of vents, tuff rings, and maybe
maar volcanoes; 3) melting of permafrost due to higher crustal heat flow
and creation of standing water surface water (lakes, ponds, streams); 4)
deposition of glass-rich, basaltic tuff in lakes and ponds; 5)reworking
of tuff by lacustrine shoreline and fluvial processes; 6) hydrothermal
alteration (palagonitization) of tuffs; 7) leaching of iron-rich
minerals in reworked tuff and re-precipitation as hematite in
spherules/concretions/nodules; 8) heat flow subsides, everything dries
out, winds return and erode lake bottom, various impacts; 9) Opportunity
lands (bunny comes to investigate); 10) speculation ensues.

By george, you may have it... Water not persistent in that scenario though,
just a flash in the pan.

Joe