Judging from your images, I think you’re correct, but now to find the source. 

Herbicide damage in gardens typically occurs from one of 3 reasons:

1. You used a sprayer which previously contained herbicide.

2. Herbicide drift occurred when nearby weeds were sprayed.

3. You used composted organic material affected with “herbicide carryover” – the most common source is composted manures obtained from a local farmer or even if obtained in a commercial package.

If you can rule out the first 2, herbicide carryover is the most likely cause.  

Unfortunately, herbicide carryover has been a widespread problem across the country for about 10  years. Such contamination has occurred with composted manures from a local farmer as well as in commercial & bulk composts (packaged & bulk) and potting mixes.

The underlying situation is this: One of several agricultural herbicides effective against broadleaf weeds in such crops as a pasture and grain fields. When livestock feed on the grass or grain, these herbicides survive the trip through the animals’ guts and are excreted in the manure which is then composted. In order for the chemicals to be inactivated, the manure must be hot composted for a year before use in gardens.

These resources describe how the damage occurred; how to verify the contamination via a simple bio-assay; and what you can do to remedy the situation in the garden.

- “Herbicide Contaminated Organic Material Kills Vegetables”- http://www.ext.colostate.edu/sam/nl/11-summer.pdf

- Page 3 of “Herbicide carryover in hay, manure, compost, and grass clippings” describes an easy bio-assay you can do: http://content.ces.ncsu.edu/herbicide-carryover

- Numerous images of various affected plants as well as informative text (via the menu at left of the page) including an easy-to-do bio-assay: (http://whatcom.wsu.edu/ag/aminopyralid/)

I now suggest gardeners do a bio-assay prior to using any composted materials from other sources to avoid unwelcome surprises.