Argentine Escoria (Chapadmalal Impactite)

Here's the general story:

At least six different impactite layers have been discovered within the last few years in the extensive loess deposits of Argentina, and another is reported in Uruguay*. The specimens presented here are from what is likely the most important of these, the Chapadmalal Impact. Dated at 3.3 million years, this event is associated with the extinction of at least 36 genera of land animals and a temperature shift that may have been of global proportions. It has been suggested that this may have been the event that triggered a climatic destabilization leading to subsequent ice-age cycles as the timing is about right.

No source crater has yet been identified, but the presence of splatted glass bombs with maximum dimensions up to 2 meters having baked soil horizons underneath suggests that the impact was very nearby. It is speculated that the retreating shoreline in this area may have destroyed the crater.

Today, the impact glass, termed "Escoria", is found in sea cliff exposures extending at least 30 kilometers eastwards from the Mar del Plata area of the Buenos Aires Province, pictured at left. Relatively fragile material eroded onto the beaches is quickly destroyed by abrasion, so it is necessary to rappel down the cliffs to obtain quality specimens.

The impactite glass reportedly contains anomalous iridium and chromium relative to the associated loessoid deposits, and like many impactites, has an exceptionally low water content relative to other natural glasses. Protolith zircon grains have been transformed into baddeleyite, implying temperatures of formation in excess of 1700°C. This temperature constraint eliminates most natural possibilities other than extraterrestrial impact. Contorted schlieren bands are evident in thin sections. Magnetic spots similar to those seen in Monturaqui glass are sparingly present, but I have not yet seen metallic fragments on cut surfaces.

The material is highly vesiculated, often with about 30% vesicles by volume (15 to 50% range). The voids are commonly irregular and show moderate vertical flattening. They are complexely interlinked, and mostly under 1 cm in maximum dimension. Where open to the surface of the specimen, the vesicles are sometimes packed with sand containing well-rounded white zinfandel-colored garnets. The glass is mostly nearly opaque, dark champagne-bottle olive green, but includes streaks of creamy yellow-green similar to Trinitite color. Most is opaque to translucent, but occasional vitreous transparent seams are present in less vesiculated material. A few specimens show blockly angular enclaves (engulfed clasts?) of denser, finely vesicular dark glass. The latter is commonly attracted to a strong magnet. There are no visible phenocrysts or inclusions.

Where surface skin is visible, is has the ropy , bubble-punctured, pahoehoe character of fluid lavas, some trinitite, and the peculiar Australian Edieowie glass.

From the distribution and maximum known size of the glass bombs, an original crater size on the order of 10 to 15 kilometers has been inferred. Investigators speculate that the responsible bolide was an order of magnitude smaller than Chicxulub, perhaps something around a one km diameter.

Because the Chapadmalal impact appears to be involved in extinctions and significant climate change, the event continues to be a topic of extensive research funded by NASA and the NSF. While not a monster killer like the K-T event, Chapadmalal may represent something approaching a lower limit on the bolide size necessary to produce global effects, and is thus significant in assessing risks posed by potential future impacts.

*7/13/04: I have just received a shipment of Escorias from Chasico, Argentina (10.1 m.y.), and some weird charcoal-bearing material from La Paloma, Uruguay. New pages coming soon---Check back or write me if you can't wait.

The following table has been fully updated on April 1, although verbal descriptions are still being inserted. The 2nd shipment begins at #59 and proceeds from largest to smallest by weight. All images are clickable, with the enlarged resulting images often showing both sides of the specimen. The enlarged images will open in new windows. CLOSE those windows to return here.

Sold items updated 2/10/2007 Most of it is gone, but several fine pieces remain---

Be sure and check out the image of an 820 gram giant following this table! It is for sale.

	#20	9.8 gms	$15Sold	individual, densely vesiculated
	#21	9.0 gms	$14sold	individual, densely vesiculated
	#22	7.5 gms	$12Sold	individual, exceptionally large vesicle
	#26	5.2 gms	$8Sold
	#28	6.3 gms	$10Sold
	#31	4.1 gms	$6Sold
	#34	1.9 gms	$6Sold
	#35	1.4 gms	$5Sold	individual
	#37	21.5 gms	$30Sold	endcut, densely vesiculated
	#39	36.7 gms	$40	endcut, extensive sand fill, local magnetic response. Big surface area
	#40	17.9 gms	$25Sold	endcut, densely vesiculated
	#41	24.0 gms	$30Sold	thick slice, densely vesiculated. Truly "escoria".
	#42	15.1 gms	$20Sold	endcut, nice flattened vesicle cross-section
	#45	16.7 gms	$20Sold	endcut, large cut surface, complex sand infill
	#46	19.8 gms	$25Sold	endcut, large vesicle
	#48	14.6 gms	$15Sold	endcut, weak local magnetic response
	#49	6.9 gms	$9Sold	endcut
	#50	7.6 gms	$10Sold	endcut
	#51	6.6 gms	$12Sold	endcut, local magnetic response, complex sand infill
	#52	6.2 gms	$10Sold	endcut, local magnetic response
	#53	7.2 gms	$10Sold	slice, local magnetic response
	#54	6.8 gms	$10Sold	slice, local magnetic response
	#56	8.7 gms	$12Sold	endcut
	#57	7.9 gms	$12Sold	endcut
	#58	3.4 gm	$8Sold	endcut, magnetic response
	#59	87.0 gms	$110	A fine big piece, with both broken surfaces and preserved surface skin patches
	#60	50.5 gms	$75Sold
	#61	41.0 gms	$50Sold	Excellent surface skin preservation
	#62	26.3 gms	$32
	#63	26.2 gms	$35Sold
	#64	21.2 gms	$30Sold
	#65	19.4 gms	$25
	#66	19.0 gms	$25
	#67	17.9 gms	$22Sold
	#68	14.2 gms	$20Sold
	#69	13.9 gms	$50Sold	Excellent skin preservation. One of my very best.
	#70	12.6 gms	$15 Sold
	#71	11.4 gms	$15Sold
	#72	11.3 gms	$18Sold
	#73	8.4 gms	$13Sold
	#74	8.3 gms	$13 Sold
	#75	7.5 gms	$12 Sold
	#76	7.3 gms	$18Sold
	#77	7.1 gms	$12Sold
	#78	6.5 gms	$13sold	Denser than most
	#79	6.4 gms	$11Sold
	#80	6.3 gms	$10Sold
	#81	5.1 gms	$9Sold
	#82	4.9 gms	$9Sold
	#83	4.7 gms	$14Sold
	#84	4.0 gms	$8 Sold
	#85	3.6 gms	$9Sold
	#86	3.5 gms	$9Sold
	#87	3.4 gms	$8Sold
	#88	3.1 gms	$7Sold
	#89	2.6 gms	$6 Sold
	#90	2.4 gms	$7Sold
	#91	2.4 gms	$5Sold
	#92	2.3 gms	$7Sold	Sold
	#93	2.1 gms	$5Sold

This big guy is the largest specimen commerically offered in the world today. At approximately 820 grams, I can offer it to serious buyers at $850. I'll provide more pictures if you are seriously interested. This is a very nice piece!

Email us at nlehrman@nvbell.net to inquire or order.

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