The Great Nevada Impact Crater Puzzle

Researchers have mapped the Alamo Breccia, an impact deposit created about 370 million years ago, into an area covering three zones and a deeper western basin.

Map of the extent of the Alamo impact (for a higher-resolution version, click here).

Numbers on the map at left and lower cross section below indicate the position of nine different types of breccias created by the Alamo impact event. Four of these are different kinds of seismites, created by impact energy transmitted through bedrock. The cross sections apply Verne Oberbeck and colleagues’ 1998 shallow marine impact model to the Alamo Breccia.

Zone 1: Breccia types 1 and 2 are interpreted as seismites that formed under the crater. Type 1 is a sandstone-filled dike-and-sill system injected into a dolomite formation 300 meters under the Alamo Breccia. Type 2 is a distinctive system of fractures and fluid transfer within the dolomite. Type 3 is breccia that eventually flowed into the newly formed crater.

Zone 2: Early shock waves propagated through the shelf limestone, leaving vertical fractures and a thin interval of sub-horizontal fluidized rock (seismite, type 4 breccia) 50 to 100 meters below the surface. This weakened interval and the fractures shaped megablocks hundreds of meters long, which laterally oscillated from subsequent seismic waves and partially self-destructed. The result is a train of discontinuous megablocks, some with spectacularly folded ends, separated by broken rock of all sizes.

Meanwhile, the impact ejecta curtain spread outward at initial speeds that easily exceeded Mach 1, pummeled the shelf and mixed with preexisting shelf limestone to form chaotic type 5 breccia. Lapilli accreted in flight, blanketed large areas, and began to harden into a bed. A transient crater and slurry rim of water and debris formed, which then collapsed outward to create tsunami 1 and inward to form waterspout 1. Tsunami 1 propagated across zone 2 and reworked ejecta and newly broken rock. Waterspout 1 collapsed to create tsunami 2. Backwash of tsunami 1 formed a second waterspout that collapsed. As many as five such cycles occurred and damped out, leaving as many as five stacked, progressively thinning-upward and graded tsunamites (type 6). During violent reworking of the shelf, the lapilli bed was dismembered and scattered among other clasts. Tsunami backwash filled the crater cavity with well-sorted debris in zone 1 (type 3).

Zone 3: The most powerful tsunami left a thin, graded bed typical of zone 3 (type 7). Seismic waves fractured the shelf and formed distinctive seismites (type 8) as far as Frenchman Mountain near Las Vegas.
Cratering processes and shelf-rim collapse shed large volumes of debris, creating the variety of deepwater breccias (type 9) still being found in offshore formations.

Although this working scenario explains the distribution of the types of Alamo Breccia, problems remain with respect to the original paleogeographic configuration of the shelf, slope and basin, and the crucial question of the location of the crater.

JEW

INFLUENCE OF ZONAL DEPOSITION - ore deposits.
Metals are deposited in a rather definite sequence. This is reflected in the changing composition of deposits in depth. Minerals deposited at higher temperature and pressure are found at greater depths than are those deposited at a lower temperature and pressure. Overlapping of deposition is common, and the occurrence of metals in reverse of the normal order is not unknown. Changing environment during deposition would probably account for this variation.

Spurr shows that the order of deposition of the metals upward from the magmatic source are similar in many of the major metal-producing districts of the world and that the usual sequence in the deposits associated with intrusive rocks in the order of deposition away from the source of the solutions are molybdenum, tungsten, gold, copper (silver), zinc, lead (silver).

Emmons shows that the metals are usually arranged in a rather definite sequence outward as well as upward from the source, and has listed the metals according to a zonal arrangement. His zonal sequence successively from the surface toward the parent Batholith is here reproduced.

A RECONSTRUCTED VEIN SYSTEM FROM SURFACE TO NEAR BATHOLITH ROOF. (After W. H. Emmons.)
BARREN.... ..... 1 .... Barren zone, chalcedony, quartz, barite, fluorite,
etc. Some veins carry a little mercury, antimony, or arsenic.
MERCURY....... 2 .... Quicksilver veins, commonly with chalcedony, marcasite, etc. Barite-fluorite veins.
ANTIMONY...... 3.... Antimony ores, stibnite often passing downward into lead, with antimonates. Many carry gold.
GOLD, SILVER... 4....Bonanza ores of precious metals. Argentite, antimony, and arsenic minerals common. Silver minerals, some copper, lead and zinc sulphides, quartz, calcite, rhodochrosite, adularia, alunite, etc.
BARREN... ... .. 5 .... Most nearly consistent barren zone, represents the bottoms of many Tertiary precious metals veins. Quartz, carbonates, etc., with pyrite and small amounts of other sulphides.
SILVER.. . .. 6....Argentite veins, complex antimony silver sulphides, stibnite, etc. Galena veins with silver. Commonly silver decreases with depth. Quartz gangue, siderite common, often increasing with depth.
LEAD ............. 7 -...Galena veins, commonly with some silver. Sphalerite, generally present, increasing with depth. Chalcopyrite common. Gangue is quartz and often carbonates (Fe, Mn, Ca) .
ZInc ....... ...... 8 .... Sphalerite veins with some lead and chalcopyrite, quartz gangue.
COPPER... ...... . 9 .... Tetrahedrite veins, commonly argentiferous, chalcopyrite present. Some pass downward into chalcopyrite. Enargite veins generally with tetrahedrite and tennantite.
COPPER.. .... 10. ...Chalcopyrite veins, generally with pyrite, often with pyrrhotite. The gangue is quartz and in some places carbonates. Some pass downward into pyrite and pyrrhotite, with a little chalcopyrite. Generally carry silver and gold.
GOLD. ...... ...... 11 .... Gold veins with quartz, pyrite, and commonly arsenopyrite and chalcopyrite.

At places, 10 and 11 are reversed.

BISMUTH... .... 12. ... Bismuthinite and native bismuth with quartz and pyrite, etc.
ARSENIC.... ..... 13 ....Arsenopyrite with chalcopyrite and often tungsten ores.
TUNGSTEN... . 14. ... Tungsten veins with quartz, pyrite, chalcopyrite, pyrrhotite, etc. Arsenopyrite is commonly present.
TIN ........-........ 15 .... Cassiterite veins with quartz, tourmaline, topaz, etc.
BARREN... ....... 16. ...Quartz with small amounts of other minerals.

There is a general agreement in the order of deposition of the , metals as listed by these authors. Spurr and many other students of ore deposits have expressed the same general conclusions. It must be emphasized, however, that in no single deposit are all of these changes observable. Erosion may have removed all but the lower zones or, as is the case with many deposits formed at shallow depths, mining operations have been carried to insufficient depth to prove or disprove the zonal deposition theory. That the metals have been deposited in an orderly sequence outward from the magmatic source is evident from the relative position of deposits in some areas. It is not uncommon to find deposits of tungsten and copper near the intrusive, and those of zinc, lead, and silver several hundreds or thousands of feet from the intrusive.